Systems and apparatus relating to reheat combustion turbine engines with exhaust gas recirculation

ABSTRACT

A power plant configured to include a recirculation loop about which a working fluid is recirculated, the recirculation loop comprising a plurality of components configured to accept an outflow of working fluid from a neighboring upstream component and provide an inflow of working fluid to a neighboring downstream component. The recirculation loop may include: a recirculation compressor; an upstream combustor; a high-pressure turbine; a downstream combustor; a low-pressure turbine; and a recirculation conduit configured to direct the outflow of working fluid from the low-pressure turbine to the recirculation compressor. The power plant may include: an oxidant compressor configured to provide compressed oxidant to both the upstream combustor and the downstream combustor; and means for extracting a portion of the working fluid from an extraction point disposed on the recirculation loop.

BACKGROUND OF THE INVENTION

This application is related to Ser. Nos. 13/444,956, 13/444,972,13/444,906, 13/444,918, 13/444,929, 13/444,948, 13/444,986, and13/444,003 filed concurrently herewith, which are fully incorporated byreference herein and made a part hereof.

This present application relates generally to combustion turbine enginesand systems related thereto. More specifically, but not by way oflimitation, the present application relates to methods, systems and/orapparatus for achieving operation at the stoichiometric point andextracting a working fluid having desired characteristics within varioustypes of combustion turbine systems having exhaust gas recirculation.

Oxidant-fuel ratio is the mass ratio of oxidant, typically air, to fuelpresent in an internal combustion engine. As one of ordinary skill inthe art will appreciate, if just enough oxidant is provided tocompletely burn all of the fuel, a stoichiometric ratio of 1 is achieved(which may be referred to herein as “operating at the stoichiometricpoint” or “stoichiometric point operation”). In combustion turbinesystems, it will be appreciated that combustion at the stoichiometricpoint may be desirable for several reasons, including lowering emissionslevels as well as performance tuning reasons. In addition, bydefinition, stoichiometric point operation may be used to provide anexhaust (which, in the case of a system that includes exhaustrecirculation, may be referred to generally as “working fluid”) that issubstantially free of oxygen and unspent fuel. More specifically, whenoperating at the stoichiometric point, the working fluid flowing throughcertain sections of the recirculation circuit or loop may consists ofsignificantly high levels of carbon dioxide and nitrogen, which, whenfed into an air separation unit, may yield substantially pure streams ofthese gases.

As one of ordinary skill in the art will appreciate, producing gasstreams of carbon dioxide and nitrogen in this manner has economicvalue. For example, the sequestration of carbon dioxide has potentialvalue given current environmental concerns relating to emission of thisgas. In addition, pure gas streams of carbon dioxide and nitrogen areuseful in many industrial applications. Also, carbon dioxide may beinjected into the ground for enhanced oil recovery. As a result, novelpower plant system configurations and/or control methods that provideefficient methods by which stoichiometric point operation may beachieved would be useful and valuable. This would be particularly trueif novel systems and methods provided effective ways by which existingpower plants using reheat and exhaust gas recirculation could achieveimproved operation via relatively minor, cost-effective modifications.Other advantages to the systems and methods of the present inventionwill become apparent to one of ordinary skill in the art given thedescription of several exemplary embodiments that is provided below.

BRIEF DESCRIPTION OF THE INVENTION

The present application thus describes a power plant configured toinclude a recirculation loop about which a working fluid isrecirculated, the recirculation loop comprising a plurality ofcomponents configured to accept an outflow of working fluid from aneighboring upstream component and provide an inflow of working fluid toa neighboring downstream component. The recirculation loop may include:a recirculation compressor; an upstream combustor positioned downstreamof the recirculation compressor; a high-pressure turbine positioneddownstream of the upstream combustor; a downstream combustor positioneddownstream of the high-pressure turbine; a low-pressure turbinepositioned downstream of the downstream combustor; and a recirculationconduit configured to direct the outflow of working fluid from thelow-pressure turbine to the recirculation compressor. The power plantmay include: an oxidant compressor configured to provide compressedoxidant to both the upstream combustor and the downstream combustor; andmeans for extracting a portion of the working fluid from an extractionpoint disposed at a predetermined location on the recirculation loop.

These and other features of the present application will become apparentupon review of the following detailed description of the preferredembodiments when taken in conjunction with the drawings and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing illustrating an exemplary configuration ofa power plant employing exhaust gas recirculation and a reheatcombustion system;

FIG. 2 is a schematic drawing illustrating an alternative configurationof a power plant employing exhaust gas recirculation and a reheatcombustion system;

FIG. 3 is a schematic drawing illustrating an alternative configurationof a power plant employing exhaust gas recirculation and a reheatcombustion system;

FIG. 4 is a schematic drawing illustrating an alternative configurationof a power plant employing exhaust gas recirculation and a reheatcombustion system;

FIG. 5 is a schematic drawing illustrating an alternative configurationof a power plant employing exhaust gas recirculation and a reheatcombustion system;

FIG. 6 is a schematic drawing illustrating an alternative configurationof a power plant employing exhaust gas recirculation and a reheatcombustion system;

FIG. 7 is a flow diagram illustrating an exemplary method of operationrelating to a power plant employing exhaust gas recirculation and areheat combustion system;

FIG. 8 is a schematic drawing illustrating an alternative configurationof a power plant employing exhaust gas recirculation and a reheatcombustion system;

FIG. 9 is a schematic drawing illustrating an alternative configurationof a power plant employing exhaust gas recirculation and a reheatcombustion system;

FIG. 10 is a schematic drawing illustrating a configuration of analternative power plant employing exhaust gas recirculation and a singlecombustion system;

FIG. 11 is a schematic drawing illustrating an alternative configurationof a power plant employing exhaust gas recirculation and a singlecombustion system;

FIG. 12 is a schematic drawing illustrating an alternative configurationof a power plant employing exhaust gas recirculation and a singlecombustion system; and

FIG. 13 is a schematic drawing illustrating an alternative configurationof a power plant employing exhaust gas recirculation and a singlecombustion system.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the figures, where the various numbers represent likeparts throughout the several views, FIGS. 1 through 13 providedschematic illustrations of exemplary power plants according toconfigurations of the present application. As will be explained infurther detail below, these power plants include novel systemarchitectures and configurations and/or methods of control that achieveperformance advantages given the recirculation of exhaust gases. Unlessotherwise stated, the term “power plant”, as used herein, is notintended to be exclusionary and may refer to any of the configurationsdescribed herein, illustrated in the figures, or claimed. Such systemsmay include two separate turbines, exhaust gas recirculation, twocombustion systems, and/or a heat-recovery steam generator.

As illustrated in FIG. 1, the power plant 9 includes a recirculationloop 10 that includes a recirculating flow of working fluid. In certainembodiments of the present invention, as illustrated in FIG. 1, therecirculation loop 10 is the means by which exhaust gas from theturbines recirculates, thereby creating a recirculating flow of workingfluid. It will be appreciated that recirculation loop 10 is configuredsuch that each of the components positioned thereon are configured toaccept an outflow of working fluid from a neighboring upstream componentand provide an inflow of working fluid to a neighboring downstreamcomponent. Note that the several components of the recirculation loop 10will be described in reference to a designated “start position 8” on theloop 10. It will be appreciated that the start position 8 is arbitraryand the function of the system could be described in another manner orin reference to another start position without substantive effect. Asshown, the start position 8 is positioned at the intake of an axialcompressor 12. As configured, the axial compressor 12 accepts a flow ofrecirculated exhaust gases from the turbines; accordingly, the axialcompressor 12 is referred to herein as “recirculation compressor 12”.Moving in a downstream direction, the recirculation loop 10 includes anupstream combustor 22, which is associated with a high-pressure turbine30, and a downstream combustor 24, which is associated with alow-pressure turbine 32. It will be appreciated that the terms used todescribe these components are purposely descriptive so that efficientdescription of the power plant 9 is possible. While the terms are notmeant to be overly limiting, it will be appreciated that the “upstream”and “downstream” designations generally refer to the direction of flowof working fluid through the recirculation loop 10 given the designatedstart position 8. Further, the “high-pressure” and “low-pressure”designations are meant to refer to the operating pressure levels in eachturbine 30, 32 relative to the other given the position of each turbineon the recirculation loop 10.

Downstream of the low-pressure turbine 32, recirculation conduit 40channels exhaust gases to the intake of the recirculation compressor 12,which thereby recirculates the exhaust gases from the turbines (or, atleast, a portion thereof). Several other components may be positioned onthe recirculation conduit 40. It will be appreciated that thesecomponents may function to deliver the exhaust gases to therecirculation compressor 12 in a desired manner (i.e., at a desiredtemperature, pressure, humidity, etc.). As shown, in variousembodiments, a heat recovery steam generator 39, a cooler 44, and ablower 46 may be included on the recirculation conduit 40. In addition,the recirculation loop 10 may include a recirculation vent 41 whichprovides a way to controllably vent an amount of exhaust from therecirculation conduit 40 such that a desirable flow balance is achieved.For example, it will be appreciated that under steady state conditionsan amount of exhaust must be vented through the recirculation vent 41that approximately equals the amount of compressed oxidant and fuelentering the recirculation loop 10 via the oxidant compressor 11 andfuel supply 20, respectively. It will be appreciated that achieving adesired balance between oxidant/fuel injected into and exhaust ventedfrom the recirculation loop 10 may be done via sensors recording theamount of compressed oxidant and fuel entering the loop 10 and theamount of exhaust exiting, as well as, temperature sensors, valvesensors, pressure sensors within the recirculation loop 10, and otherconventional means and systems.

The power plant 9 may include an oxidant compressor 11, which, unlikethe recirculation compressor 12, is not fully integrated into therecirculation loop 10. As provided below, the oxidant compressor 11 maybe an axial compressor that is configured to inject compressed air orother oxidant at one or more locations within the recirculation loop 10.In most applications, the oxidant compressor 11 will be configured tocompress air. It will be appreciated that, in other embodiments, theoxidant compressor 11 may be configured to supply any type of oxidantwhich could be pressurized and injected into the combustion system. Forexample, the oxidant compressor 11 could compress a supply of air dopedwith oxygen. The recirculation compressor 12, on the other hand, isconfigured to compress recirculated exhaust gases from the turbines 30,32. When necessary, a booster compressor 16 may be provided to boost thepressure of the discharge of the oxidant compressor 11 before it isinjected into the recirculation loop 10 so that a preferable injectionpressure is achieved. In this manner, the compressed oxidant may beeffectively delivered to one or more combustors.

The oxidant compressor 11 and the recirculation compressor 12 may bemechanically coupled by a single or common shaft 14 that drives both. Agenerator 18 may also be included on the common shaft 14, while thehigh-pressure turbine 30 and the low-pressure turbine 32 drive thecommon shaft 14 and the loads attached thereto. It will be appreciatedthat the present invention may be employed in systems having shaftconfigurations different than the exemplary common shaft configuration14 illustrated in the figures. For example, multiple shafts may be used,each of which may include one of the turbines and one or more of theload elements (i.e., one of the compressors 11, 12 or the generator 18).Such a configuration may include concentric shafts or otherwise.

In exemplary embodiments, the combustion system of the power plant 9, asshown, includes an upstream combustor 22 and, downstream of that, adownstream combustor 24. It will be appreciated that, as discussed inmore detail below, the upstream combustor 22 and the downstreamcombustor 24 may include any type of conventional combustors, combustionsystems and/or reheat combustors, and the chosen terminology refers onlyto relative positioning on the recirculation loop 10 (given thedesignated start position 8 and direction of flow). Typically, asdepicted in FIG. 1 and discussed in more detail below, the upstreamcombustor 22 operates by injecting into the recirculation loop 10combustion gas resulting from a fuel being combusted in a can combustoror other type of conventional combustor. Alternatively, certaincombustion systems operate by direct fuel injection. Upon injection, theinjected fuel combusts within the recirculation loop 10. Either of thesemethods generally increases the temperature and the kinetic energy ofthe working fluid, and either of the combustor types may be employed asthe upstream combustor 22 or the downstream combustor 24. A fuel supply20 may supply fuel, such as natural gas, to the upstream combustor 22and the downstream combustor 24.

More specifically, the upstream combustor 22 may be configured to acceptthe flow of compressed oxidant from the oxidant compressor 11 and a fuelfrom the fuel supply 20. In this mode of operation, the upstreamcombustor 22 may include one or more cans or combustion chambers withinwhich fuel and oxidant are brought together, mixed, and ignited suchthat a high energy flow of pressurized combustion gases is created. Theupstream combustor 22 then may direct the combustion gases into thehigh-pressure turbine 30, where the gases are expanded and workextracted. The downstream combustor 24 may be configured to addenergy/heat to the working fluid at a point downstream of thehigh-pressure turbine 30. As shown in the embodiment of FIG. 1, thedownstream combustor 24 may be positioned just upstream of thelow-pressure turbine 32. As stated, the downstream combustor 24 isso-called because it adds heat/energy to the flow of working fluid at apoint downstream of the upstream combustor 22.

As one of ordinary skill in the art will appreciate, certain operationaladvantages may be achieved using a dual combustion or reheat system suchas those described above. These advantages include, among otherthings: 1) fuel flexibility; 2) improved emissions; 3) lower overallfiring temperatures; 4) less cooling and sealing requirements; 5) longerpart life; and 6) use of less expensive materials due to lower firingtemps. Accordingly, improving the operation of power plants that includereheat systems, as provided by the present invention, widens thepotential usage of reheat systems and the realization of the advantagesthese systems typically provide.

As mentioned, the power plant 9 further includes recirculation conduit40. The recirculation conduit 40, in general, forms the flow path bywhich exhaust from the turbines is recirculated, thereby completing therecirculation loop 10. More specifically, the recirculation conduit 40directs the exhaust from the low pressure turbine 32 on a path that endsat the intake of the recirculation compressor 12. It will be appreciatedthat the recirculation conduit 40 may circulate the exhaust throughseveral components along the way, including, as indicated in FIG. 1, aheat-recovery steam generator 39, a cooler 44, and a blower 46. (Notethat, to avoid unnecessary complexity, the heat-recovery steam generator39 has been represented in a simplified form in FIG. 1.) Those ofordinary skill in the art will appreciate that the heat-recovery steamgenerator 39 of the present invention may include any type of system inwhich combustion exhaust from one or more combustion turbines is used asthe heat source for the boiler of a steam turbine.

Downstream of the heat-recovery steam generator 39, the cooler 44 may bepositioned such that gases flowing through the recirculation conduit 40flow through it. The cooler 44 may include a direct contact cooler orother conventional heat exchanger that suffices for this function, andmay operate by extracting further heat from the exhaust gases such thatthe exhaust gases enter the recirculation compressor 12 at a desired orpreferred temperature. The cooler 44 may also provide means by whichhumidity levels within the recirculated gases is controlled topreferable levels. That is, the cooler 44 may extract water from theflow through cooling it, which thereby lowers the humidity level of therecirculated gases upon the gases being heated to the temperature of theflow before entering the cooler. As illustrated in FIG. 1, the blower 46may be located downstream of the cooler 44; however, as one of ordinaryskill in the art will appreciate, this order may be reversed. The blower46 may be of a conventional design. The blower 46 may function to moreefficiently circulate the exhaust gases through the recirculationconduit 40 such that the gases are delivered to the intake of therecirculation compressor 12 in a desired manner.

The power plant 9 may include several types of conduits, pipes, valves,sensors and other systems by which the operation of the power plant 9 iscontrolled and maintained. It will be appreciated that all valvesdescribed herein may be controlled to various settings that affect theamount of fluid passing through the conduit. As already described, therecirculation conduit 40 recirculates exhaust gases from the turbines30, 32 to the intake of the recirculation compressor 12, therebyproviding a recirculating flow path for the working fluid. In addition,as indicated in FIG. 1, a first oxidant conduit 52 may be provided thatdirects the compressed oxidant from the oxidant compressor 11 to theupstream combustor 22. The first oxidant conduit 52 may include anoxidant valve 54 that controls the flow of oxidant through this conduit.The first oxidant conduit 52 further may include the booster compressor16, which, as described in more detail below, may be used to boost thepressure of the compressed oxidant within this conduit. The firstoxidant conduit 52 may further include a vent valve 56. The vent valve56 provides means by which a portion of the compressed oxidant movingthrough the first oxidant conduit 52 is vented to atmosphere. Asindicated in FIG. 1, certain embodiments of the present inventionoperate by providing a flow of compressed oxidant from the oxidantcompressor 11 to the upstream combustor 22, but not the downstreamcombustor 24. In other embodiments, such as those shown in FIGS. 2through 5, the present invention operates by providing a flow ofcompressed oxidant from the oxidant compressor 11 to the upstreamcombustor 22 and the downstream combustor 24. In still otherembodiments, the present invention operates by providing a flow ofcompressed oxidant from the oxidant compressor 11 to the downstreamcombustor 22 but not the upstream combustor 24. This type of system, forexample, is represented in FIGS. 2 and 4 when the oxidant valve 54 onthe first oxidant conduit 52 is completely shut (i.e., set so that noflow from the oxidant compressor 11 is allowed to pass therethrough).

The fuel supply 20 may include two supply conduits that provide fuel tothe upstream combustor 22 and/or the downstream combustor 24. As shown,a fuel valve 58 controls the amount of fuel being delivered to theupstream combustor 22, while another fuel valve 59 controls the amountof fuel being delivered to the downstream combustor 24. It will beappreciated that, though not shown in the figures, the fuel typesdelivered to the upstream combustor 22 and the downstream combustor 24do not have to be the same, and that the use of different fuel types maybe advantageous given certain system criteria. In addition, as discussedin more detail below, the fuel valve 58 and the fuel valve 59 may becontrolled so that fuel is delivered to only one of the two combustors22, 24. More specifically, in certain embodiments, the fuel valve 58 maybe completely shut so that fuel is not delivered to the upstreamcombustor 22. In this case, as discussed in more detail below, bothcombustors 22, 24 may operate per the fuel delivered to the downstreamcombustor 24. Similarly, in certain embodiments, the fuel valve 59 maybe completely shut so that fuel is not delivered to the downstreamcombustor 22. In this case, as discussed in more detail below, bothcombustors 22, 24 may operate per the fuel delivered to the upstreamcombustor 22. It will be appreciated that systems described herein asoperating with a valve that is shut completely is intended to coversystem configurations where the conduit on which the shut valve ispositioned is omitted altogether.

An extraction point 51 comprises the point at which gases are extractedfrom the working fluid. In preferred embodiments, the extraction point51 is positioned on the recirculation loop 10 such that carbon dioxide(CO₂) and/or nitrogen (N₂) may be efficiently extracted. Given certainmodes of operation and system control, the system architecture of thepresent invention allows for such extraction to occur at a positionthat, as illustrated in FIG. 1, is upstream of both the high-pressureturbine 30 and the upstream combustor 22. More specifically, as shown,the extraction point 51 may be located at a position that is justupstream of the combustion reaction in the upstream combustor 22. Theextraction point 51 may include conventional extracting means by which aportion of the gases within the working fluid are diverted into aconduit and, thereby, removed from the recirculation loop 10. Anextracted gas valve 61 may be provided to control the amount of workingfluid that is extracted. Downstream of the extracted gas valve 61, theconduit may deliver an extracted gas supply 62 to one or more downstreamcomponents (not shown). In preferred embodiments, the extracted gassupply 62 may be directed to a separation system (not shown) thatseparates the carbon dioxide from the nitrogen per conventional means.As stated, after separation, these gases may be used in many types ofindustrial applications, such as, for example, applications in the foodand beverage industry.

Branching from the conduit that connects to the extraction point 51, aturbine bypass conduit 63 also may be included that provides a pathwaythat bypasses each of the turbines 30, 32. The turbine bypass conduit 63is provided for startup situations, and, because it does notmeaningfully impact the function of the present invention, will not bediscussed further.

In other embodiments, the extraction point 51 may be located indifferent locations within the recirculation loop 10 of FIG. 1. Asdescribed in more detail below (particularly with regard to FIGS. 5 and6), the architecture and control methods provided herein teach efficientand effective means by which one of the combustors 22, 24 may beoperated at or near the stoichiometric point or a preferredstoichiometric ratio. That is, the fuel and oxidant supply within thepower plant 9 may be controlled in such a way that, once the oxidant andfuel have adequately mixed, ignited and combusted within one of thecombustors 22, 24, an exhaust that is free or substantially free ofoxygen and unspent fuel is produced. In this condition, the exhaustconsists of high levels of carbon dioxide and nitrogen, which may beeconomically extracted for use in other applications. As stated,“operation at the stoichiometric point” or “stoichiometric pointoperation” refers to operation at, near or within an acceptable ordesired range about the stoichiometric point. It will be appreciatedthat “stoichiometric point” may also be referred to a stoichiometricratio of 1, as it is said to include a 1-to-1 ratio of fuel and oxidant.It will further be appreciated that ratios that are greater than 1 aredescribed as containing excess oxidant, while ratios less than 1 aredescribed as containing excess fuel. It will be appreciated that,depending on the limitations of a particular power plant, the desiredproperties of the extracted working fluid, as well as other criteria,stoichiometric point operation may refer to stoichiometric operationwithin a range about the stoichiometric point or, put another way, astoichiometric ratio of 1. Accordingly, in certain embodiments,“stoichiometric point operation” may refer to operation within the rangeof stoichiometric ratios defined between 0.75 and to 1.25. In morepreferable embodiments, “stoichiometric point operation” may refer tooperation within the range of stoichiometric ratios defined between 0.9and to 1.1. In still more preferable embodiments, “stoichiometric pointoperation” may refer to operation that is substantially at or very closeto a stoichiometric ratio of 1. Finally, in other preferableembodiments, “stoichiometric point operation” may refer to operationwithin the range of stoichiometric ratios defined between approximately1.0 and to 1.1.

It will be appreciated that if one of the combustors 22, 24 is operatedat the stoichiometric point (i.e., a stoichiometric ratio of 1 or withinone of the predefined ranges described above or another desired range),the exhaust downstream of the combustor is substantially devoid ofunspent fuel and oxygen, and consists substantially of carbon dioxideand nitrogen gas (and/or some other desirable gaseous characteristic),which may be economically extracted. As a result of this, pursuant toembodiments of the present invention, the extraction point 51 generallymay be located at any point on the recirculation loop 10 that isboth: 1) downstream of the whichever combustor 22, 24 is operating atthe stoichiometric point and 2) upstream of the other combustor 22, 24.(It will be appreciated by those of ordinary skill in the art that“upstream of the other combustor”, as used herein, means upstream of thepoint within the combustor at which oxidant and/or fuel actually entersthe recirculation loop 51, and that, because of this, “upstream of theother combustor” may include areas that might be construed as within the“other combustor” but which are also upstream of the position at whichoxidant and/or fuel is injected into the flow of working fluid, such as,for example, certain areas within a combustor head-end. In aconfiguration like FIG. 1, assuming that the fuel input of thedownstream combustor 24 is controlled to produce combustion at (orsubstantially at) the stoichiometric point, the extraction point 51 maybe located at any point within a range defined between the downstreamcombustor 24 and, proceeding in a downstream direction, the upstreamcombustor 22. In one preferred embodiment, as illustrated in FIG. 1, theextraction point may be located within this range at the discharge ofthe recirculation compressor 12. It will be appreciated that thislocation provides extracted gas that is highly pressurized, which may beadvantageous in certain downstream uses.

The power plant 9 may further include one or more sensors 70 thatmeasure operating parameters, settings, and conditions within thecomponents and various conduits of the system. One such sensor may be asensor for detecting excess oxidant 64, such as, for example, aconventional oxygen sensor. The sensor for detecting excess oxidant 64may be positioned just upstream of the extraction point 51 and maymeasure at predefined intervals the oxygen content of the exhaust orworking fluid flowing through the recirculation loop 10. Positionedthusly, the sensor for detecting excess oxidant 64 may be well situatedto test the working fluid for oxidant content, which may provideinformation as to stoichiometric ratio within the combustor directlyupstream of the sensor for detecting excess oxidant 64 and/or whetherextraction of the working fluid would yield a gas supply that issuitably free of oxidant and unspent fuel. It will be appreciated thatthe sensor for detecting excess oxidant 64 may be positioned within arange on the recirculation loop 10 that is defined between theextraction point 51 and, proceeding in the upstream direction, the firstcombustor 22, 24 that is encountered. It will be appreciated that, giventhe positioning of the extraction point 51, the first combustor 22, 24encountered in the upstream direction is the combustor 22, 24 which isbeing controlled at the preferred stoichiometric ratio. In this manner,the sensor for detecting excess oxidant 64 may be used to determine thecurrent desirability of extracting gas from the recirculation loop 10.As described in more detail below, the system may include other sensors70 that measure a host of process variables that may relate to any ofthe components of the system. Accordingly, the figures indicate aplurality of sensors 70 at exemplary locations about the power plant 9.As one of ordinary skill in the art will appreciate, conventionalsystems typically include many sensors other than just those representedin the several figures, and, further, that those other sensors may belocated in other locations within the system than just those indicated.It will be appreciated that these sensors 70 may electronicallycommunicate their readings with the control unit 65 and/or functionpursuant to instructions communicated to them by the control unit 65.One such sensor 70 that could be used either together or interchangeablywith the sensor for detecting excess oxidant 64 is a sensor that detectsthe presence of unspent fuel in the exhaust. Teamed with the sensor fordetecting excess oxidant 64, a sensor for detecting unspent fuel 70could provide measurements from which the stoichiometric ratio in theupstream combustor 22, 24 could be determined as well as the currentsuitability of extracting working fluid. Those skilled in the art willappreciate that other sensors may be used to collect data concerning thestoichiometric properties of the combustion occurring within thecombustors. For example, a CO sensor and a humidity sensor may be used.

The power plant 9 may further include a control unit 65 that functionsaccording to certain embodiments described herein. It will beappreciated that the control unit 65 may include an electronic orcomputer implemented device that takes in data from sensors and othersources regarding plant operational parameters, settings, andconditions, and, pursuant to algorithms, stored data, operatorpreferences, etc., controls the settings of the various mechanical andelectrical systems of the power plant 9 such that desired modes ofoperation are achieved. For example, the control unit 65 may control thepower plant 9 such that stoichiometric operation or operation at apreferred stoichiometric ratio is achieved in one of the combustors 22,24. It will be appreciated that the control mechanism may achieve thisobjective by balancing the fuel and oxidant injected into the either theupstream or downstream combustor 22, 24, as well as taking into accountany excess oxidant or unspent fuel from the other of the two combustor22, 24 that travels within the recirculating working fluid. Oncestoichiometric operation is achieved, the control unit 65 may controlthe extracted gas valve 61 such that extraction takes place at a desiredrate and for a desired period of time or until changing conditions makethe extraction no longer suitable. The settings of the various valvesdescribed above that govern the flow of working fluid, extraction ofgases, fuel consumption, etc. may be controlled pursuant to electricalsignals, which may be sent via wired or wireless communicationconnections, received from the control unit 65.

In use, the power plant 9 according to an exemplary embodiment mayoperate as follows. The rotation of blades within oxidant compressor 11compresses oxidant that is supplied, via the first oxidant conduit 52 tothe upstream combustor 22. Before reaching the upstream combustor 22,the booster compressor 16 may be provided in some embodiments. Thebooster compressor 16 may be used to increase the pressure of theoxidant being supplied by the oxidant compressor 11 to a level that isadequate or preferable for injection into the upstream combustor 22. Inthis manner, the flow of compressed oxidant may be joined within theupstream combustor 22 with a flow of compressed exhaust gases, which issupplied to the combustor from the recirculation compressor 12. It willbe appreciated that successfully bringing together two such flows withinthe upstream combustor 22 may be accomplished in several ways and that,depending on how the flows are introduced within the upstream combustor22, suitable pressure levels for each may vary. The present inventionteaches methods and system configurations by which pressure levels maybe controlled such that the flows may be combined in a suitable manner,while avoiding avoidable aerodynamic losses, backflow, and otherpotential performance issues.

Accordingly, the upstream combustor 22 may be configured to combine theflow of compressed oxidant from the oxidant compressor 11 with the flowof compressed exhaust gases from the recirculation compressor 12 andcombust a fuel therein, producing a flow of high-energy, pressurizedcombustion gases. The flow of combustion gases then is directed over thestages of rotating blade within the high-pressure turbine 30, whichinduces rotation about the shaft 14. In this manner, the energy of thecombustion gases is transformed into the mechanical energy of therotating shaft 14. As described, the shaft 14 may couple thehigh-pressure turbine 30 to the oxidant compressor 11 so that therotation of the shaft 14 drives the oxidant compressor 11. The shaft 14further may couple the high-pressure turbine 30 to the recirculationcompressor 12 so that the rotation of the shaft 14 drives therecirculation compressor 12. The shaft 14 also may couple thehigh-pressure turbine 30 to the generator 18 so that it drives thegenerator 18 as well. It will be appreciated that the generator 18converts the mechanical energy of the rotating shaft into electricalenergy. Of course, other types of loads may be driven by thehigh-pressure turbine 30.

The working fluid (i.e., the exhaust from the high-pressure turbine 30)then is directed to the low-pressure turbine 32. Before reaching thelow-pressure turbine 32, the downstream combustor 24 adds heat/energy tothe working fluid flowing through the recirculation loop 10, asdescribed above. In the embodiment of FIG. 1, the downstream combustor24 is configured to combust a fuel within the exhaust from thehigh-pressure turbine 30. In alternative embodiments, as shown in FIGS.2-6 and discussed in more detail below, the downstream combustor 24 maybe configured to combine a flow of compressed oxidant from the oxidantcompressor with the flow of exhaust gases from the high-pressure turbine30 and combust a fuel therein, producing a flow of high-energy,pressurized combustion gases. The working fluid then is directed overthe stages of rotating blades within the low-pressure turbine 32, whichinduces rotation about the shaft 14, thereby transforming the energy ofthe combustion gases into the mechanical energy of the rotating shaft14. As with the high-pressure turbine 30, the shaft 14 may couple thelow-pressure turbine 32 to the oxidant compressor 11, the recirculationcompressor 12, and/or the generator 18. In certain embodiments, thehigh-pressure turbine 30 and the low-pressure turbine 32 may drive theseloads in tandem. In other embodiments, concentric shafts may be usedsuch that the high-pressure turbine 30 drives part of the load on one ofthe concentric shafts, while the low-pressure turbine 32 drives theremaining load on the other. Additionally, in other systemconfigurations, the high-pressure turbine 30 and the low-pressureturbine 32 may drive separate, non-concentric shafts (not shown).

From the low-pressure turbine 32, recirculation conduit 40 may form aflow path that completes the recirculation loop 10 of the presentinvention. This flow path, ultimately, delivers the exhaust gases fromthe turbines 30, 32 to the intake of the recirculation compressor 12. Aspart of this recirculation conduit 40, the exhaust gases may be used bythe heat-recovery steam generator 39. That is, the exhaust gases mayprovide a heat source for the boiler that drives a steam turbine whichreceives steam from the heat-recovery steam generator 39. Downstream ofthat, the exhaust gases may be further cooled by the cooler 44 as wellas being passed through a blower 46. The cooler 44 may be used to lowerthe temperature of the exhaust gases so that they are delivered to theintake of the recirculation compressor 12 within a desired temperaturerange. The blower 46 may assist in circulating the exhaust gases throughthe recirculation loop 10. It will be appreciated that the heat recoverysteam generator 39, the cooler 44 and the blower 46 may includeconventional components and be operated pursuant to conventionalmethods.

In regard to the operation of the control unit 65, it will beappreciated that it may include an electronic or computer implementeddevice that takes in data regarding plant operational parameters andconditions, and, pursuant to algorithms, stored data, operatorpreferences, etc., controls the settings of the various mechanical andelectrical systems of the power plant 9 such that desired modes ofoperation are achieved—for example, achieving operation at orsubstantially at the stoichiometric point. The control unit 65 mayinclude control logic specifying how the mechanical and electricalsystems of the power plant 9 should operate. More specifically, and inaccordance with certain embodiments of the present application, thecontrol unit 65 typically includes programmed logic that specifies howcertain operating parameters/stored data/operator preferences/etc.should be monitored and, given certain inputs from the monitored data,how the various mechanical and electrical systems of the power plant 9,such as those described above, should be operated. The control unit 65may control the operation of the various systems and devicesautomatically in response to the dictates of the control logic, or, incertain instances, may seek operator input before actions are taken. Asone of ordinary skill in the art will appreciate, such a system mayinclude multiple sensors, devices, and instruments, some of which arediscussed above, that monitor relevant operational parameters. Thesehardware devices may transmit data and information to the control unit65, as well as being controlled and manipulated by the control unit 65.That is, pursuant to conventional means and methods, the control unit 65may receive and/or acquire data from the systems of the power plant 9,process the data, consult stored data, communicate with the operators ofthe power plant 9, and/or control the various mechanical and electricaldevices of the system pursuant to a set of instructions or logic flowdiagrams, which, as one of ordinary skill in the art will appreciate,may be made part of a software program that is operated by control unit65, and which may include aspects relating to embodiments of the presentinvention. In short, the control unit 65 may control operation of thepower plant 9 such that it operates at the stoichiometric point and,while operating thusly, extracts a supply of combustion exhaust that issubstantially devoid of oxygen and unspent fuel. Discussion below, inrelation to FIG. 7, relates to logic flow diagrams according to thepresent invention for operating the systems described herein at thestoichiometric point and extraction of desirable exhaust gas. It will beappreciated that these logic flow diagrams may be used by the controlunit for such purposes.

FIGS. 2 through 6 provide embodiments of the present invention thatinclude alternative system configurations. It will be appreciated thatthese configurations present alternative strategies for injectingoxidant from the oxidant compressor 11 into the recirculation loop 10,delivering fuel to the combustion systems, and the manner in whichexhaust gases may be extracted. Each of these alternatives offerscertain advantages, including the manner in which stoichiometricoperation may be achieved and maintained. It will be appreciated thatthese alternatives are exemplary and not intended to provide anexhaustive description of all possible system configurations which mightfall within the scope of the appended claims. In addition, while FIGS. 2through 6 illustrate both fuel and oxidant being delivered to each ofthe upstream and the downstream combustor 22, 24, it will be appreciatedthat certain embodiments described below function in systems in whichoxidant is delivered to only one of the upstream and downstreamcombustors 22, 24 and/or systems in which fuel is delivered to only oneof the upstream and downstream combustors 22, 24. Examples of any ofthese systems may be constructed via control of the various valves 54,58, 59, 68 that deliver oxidant and fuel to the combustors 22, 24.

FIG. 2 through 4 provide embodiments that include a second oxidantconduit 67 and oxidant valve 68, which together may be used to supply acontrolled compressed oxidant amount (which like the first oxidantconduit 52 is derived from the oxidant compressor 11) to the downstreamcombustor 24. As shown in FIGS. 2 and 3, the second oxidant conduit 67may branch from the first oxidant conduit 52, which means that thecompressed oxidant for each is drawn from the same supply point from theoxidant compressor 11. In FIG. 2, the branching occurs such that aconnection with the first oxidant conduit 52 occurs upstream of theoxidant valve 54 and booster compressor 16 of the first oxidant conduit52. In this case, the second oxidant conduit 67 thereby bypasses thebooster compressor 16. This may be useful in creating flows of differingpressures levels within the first oxidant conduit 52, which would have ahigher pressure due to the booster compressor 16 than that within thesecond oxidant conduit 67. As the first oxidant conduit 52 providescompressed oxidant to a point on the recirculation loop 10 upstream ofthe second oxidant conduit 67, this configuration allows for anefficient means by which the pressure in each may be controlled to apressure level that is appropriate for injection at the differentlocations. In FIG. 3, the branching occurs downstream of the oxidantvalve 54 of the first oxidant conduit 52. More specifically, thebranching of the second oxidant conduit 52 occurs between the oxidantvalve 54 of the first oxidant conduit 52 (which may be positioneddownstream of the booster compressor 16, as shown) and combustor 22.

As illustrated in FIG. 4, the second oxidant conduit 67 may also beindependent of the first oxidant conduit 52. As shown, in this instance,the second oxidant conduit 67 may extend from an extraction point withinthe oxidant compressor 11. The extraction point for the second oxidantconduit may be located at one of the stages that is upstream of theposition where the first oxidant conduit 52 derives its flow ofcompressed oxidant, which, for example, may be located in the compressordischarge casing. More specifically, the extraction point may beconfigured to bleed compressed oxidant at an intermediate stage withinthe oxidant compressor 11. With the first oxidant conduit 52 drawingfrom the compressor discharge casing or in proximity thereto, thisarrangement results in a higher pressure flow of compressed oxidantthrough the first oxidant conduit 52 than that in the second oxidantconduit 67. It again will be appreciated that this configuration allowsthe first and second oxidant conduits 52, 67 to have differing pressurelevels without the need of including a booster compressor 16. As before,the pressure differential may be useful in that the pressure of thecompressed oxidant may be matched to the pressure at the position on therecirculation loop 10 it is used.

FIGS. 5 and 6 provide differing strategies for locating the extractionpoint 51 given the fact that both combustors 22, 24 receive a supply ofcompressed oxidant from the oxidant compressor 11. It will beappreciated that configuring the system to have two points at whichoxidant/fuel are combusted provides new alternatives for producingoperation at the stoichiometric point (note that, as stated, this refersto operation within a desired range about or near the stoichiometricpoint), and, thus, differing locations (as provided in FIGS. 5 and 6) atwhich working fluid may be extracted. As described, the architecture andcontrol methods provided herein teach efficient and effective means bywhich power plants may be operated at the stoichiometric point. The fueland oxidant supply to the power plant 9 may be controlled in such waythat, once the oxygen (from the injected oxidant) and fuel haveadequately mixed, ignited and combusted, an exhaust that issubstantially free of oxygen and unspent fuel is produced. As a resultof this, pursuant to embodiments of the present invention, theextraction point 51 may be located at any point on the recirculationloop 10 that has exhaust derived from stoichiometric point operation. Asdescribed above in relation to the configuration of FIG. 1, thisgenerally means that the extraction point may be located at any positionon the recirculation loop 10 that is both: 1) downstream of thecombustor 22, 24 which is being operated at the stoichiometric point;and 2) upstream of the other combustor 22, 24. It will be appreciatedthat more than one extraction point within this range may be provided,and that this arrangement may be useful where different pressure levelsare useful for a plurality of extracted gas supplies.

FIG. 5 illustrates an exemplary configuration having an extraction point51 that is positioned near the aft end of the high-pressure turbine 30.It will be appreciated that this extraction point 51 may prove effectivewhen the upstream combustor 22 operates at the stoichiometric point.Given the principles discussed above and assuming this operation,possible extraction points 51 constitute a range defined between theupstream combustor 22 and, proceeding in the downstream direction, thedownstream combustor 24. That is, pursuant to embodiments of the presentinvention, the power plant 9 may be controlled such that the combinedeffect of the oxidant and fuel introduced within the combustors 22, 24produces combustion within the upstream combustor 22 at a preferredstoichiometric ratio, which thereby creates a range of positionsdownstream of the upstream combustor 22 in which extraction of workingfluid having desired characteristics may be achieved.

FIG. 6 illustrates an exemplary configuration having an extraction point51 that is positioned just upstream of the heat recovery steam generator39. It will be appreciated that this extraction point 51 may proveeffective when the downstream combustor 24 operates at thestoichiometric point. Given the principles discussed above and assumingthis operation, possible extraction points 51 constitute a range definedbetween the downstream combustor 24 and, proceeding in the downstreamdirection, the upstream combustor 22. That is, pursuant to embodimentsof the present invention, the power plant 9 may be controlled such thatthe combined effect of the oxidant and fuel introduced within thecombustors 22, 24 produces combustion within the downstream combustor 24at a preferred stoichiometric ratio, which thereby creates a range ofpositions downstream of the downstream combustor 24 in which extractionof the working fluid having desired characteristics may be achieved.

FIG. 7 illustrates a logic flow diagram 100 for a method of operatingthe power plant 9 according to an exemplary embodiment of the presentinvention. As one of ordinary skill in the art will appreciate, thelogic flow diagram 100 is exemplary and includes steps which may not beincluded in the appended claims. Further, any function described abovein relationship to the several components of the system is incorporatedinto the discussion below where necessary or possible to aid in thecarrying out of the specified steps. The logic flow diagram 100 may beimplemented and performed by the control unit 65. In some embodiments,the control unit 65 may comprise any appropriate high-poweredsolid-state switching device. The control unit 65 may be a computer;however, this is merely exemplary of an appropriate high-powered controlsystem, which is within the scope of the present application. In certainembodiments, the control unit 65 may be implemented as a single specialpurpose integrated circuit, such as ASIC, having a main or centralprocessor section for overall, system-level control, and separatesections dedicated to performing various different specificcombinations, functions and other processes under control of the centralprocessor section. It will be appreciated by those skilled in the artthat the control unit also may be implemented using a variety ofseparate dedicated or programmable integrated or other electroniccircuits or devices, such as hardwired electronic or logic circuitsincluding discrete element circuits or programmable logic devices. Thecontrol unit 65 also may be implemented using a suitably programmedgeneral-purpose computer, such as a microprocessor or microcontroller,or other processor device, such as a CPU or MPU, either alone or inconjunction with one or more peripheral data and signal processingdevices. In general, any device or similar devices on which a finitestate machine capable of implementing the logic flow diagram 100 may beused as the control unit 65.

It will be appreciated that, in one possible environment, the controlunit 65 may include a General Electric SPEEDTRONIC™ Gas Turbine ControlSystem, such as is described in Rowen, W. I., “SPEEDTRONIC™ Mark V GasTurbine Control System”, GE-3658D, published by GE Industrial & PowerSystems of Schenectady, N.Y. The control unit 65 may be a computersystem having a processor(s) that executes programs to control theoperation of the gas turbine using sensor inputs and instructions fromhuman operators. The programs executed by the control unit 65 mayinclude scheduling algorithms for regulating the components of the powerplant 9. The commands generated by the control unit 65 may causeactuators within any of the components to, for example, adjust valvesbetween the fuel supply and combustors 22, 24 that regulate the flow andtype of fuel, inlet guide vanes on the compressors 11, 12, and othercontrol settings on the turbine 30, 32. Further, the control unit 65 mayregulate the power plant 9 based, in part, on algorithms stored incomputer memory of the control unit 65. These algorithms, for example,may enable the control unit 65 to maintain emission levels in exhaust towithin certain predefined limits, to maintain the combustor firingtemperature to within predefined temperature limits, or another maintainoperational parameter within a predefined range.

Returning to FIG. 7, one of ordinary skill in the art will appreciatethat, in general, flow diagram 100 illustrates an example of how afeedback loop may be structured to provide an iterative process forcontrolling stoichiometry within one of the combustors and/or extractionlevel of exhaust having desired characteristics. It will be appreciatedthat the several steps of such a process may be described in manydifferent ways without deviating from the central idea of the processset forth herein. The control methods described herein may beimplemented via a feedback loop that is used in conjunction with controlalgorithms, such as a PID control algorithm, though other controlalgorithms also may be used.

Logic flow diagram 100 may begin at a step 102, which includesmonitoring and measuring the operating conditions and process variables(which will be referred to generally as “process variables”) of thepower plant 9. Process variables, as used herein, represent the currentstatus of the system or process that is being controlled. In this case,process variables may include any operating parameter that may bemeasured by any type of sensor. More specifically, at step 102, thecontrol unit 65, pursuant to any of the methods discussed above or anyconventional systems (either current or developed in the future), mayreceive, monitor, and record data relating to the operation of the powerplant 9. The operation of the power plant 9 and the several componentsrelated thereto may be monitored by several sensors 70 detecting variousconditions of the system and environment. For example, one or more ofthe following process variable may be monitored by the sensors 70:temperature sensors may monitor ambient temperature surrounding theplant 9, inlet and discharge temperatures of the compressors 11, 12,exhaust temperature and other temperature measurements along the hot-gaspath of the turbines 30, 32, pressure sensors may monitor ambientpressure, and static and dynamic pressure levels at the inlet and outletof the compressors 11, 12, and exhaust of the turbines 30, 32, as wellas at other locations in the gas stream. Sensors 70 further may measurethe extraction level at the extraction point 51, fuel flow to each ofthe combustors 22, 24, gas composition within the recirculated exhaustgas or working fluid (which may include the sensor for detecting excessoxidant 64 as well as other sensors that measure levels of unspent fuelor CO or other gases within the exhaust gas), temperature and pressureof the recirculated exhaust gas along the recirculation conduit 10,including parameters relating to the operation of the heat recoverysteam generator 39, the cooler 44, and the blower 46. The sensors 70 mayalso comprise flow sensors, speed sensors, flame detector sensors, valveposition sensors, guide vane angle sensors, and the like that sensevarious parameters pertinent to the operation of power plant 9, whichmay include oxidant flow characteristics through the first oxidantconduit 52 and the second oxidant conduit 67. It will be appreciatedthat the system may further store and monitor certain “specifiedset-points” that include operator preferences relating to preferred orefficient modes of operation. It further will be appreciated that themeasuring, monitoring, storing and/or recording of process variablesand/or specified set-point may occur continuously or at regularintervals, and that updated or current data may be used throughout anyof the several steps of logic flow diagram 100 whether or not there is adirect line in FIG. 7 connecting step 102 to the other steps. From step102, the process may continue to step 104.

At a step 104, the method may determine whether whichever combustor 22,24 is configured to operate at a preferred stoichiometric ratio (whichmay include a range of suitable stoichiometric ratios) is, in fact,operating at the preferred stoichiometric ratio. It will be appreciatedthat this may be accomplished by comparing measured process variables,calculating current conditions, and comparing current conditions tospecified set-points. If it is determined that this mode of operation isoccurring, the method may proceed to step 106. If it is determined thatthis mode of operation is not occurring, the method may proceed to step114.

It will be appreciated that the determination as to whether the relevantcombustor 22, 24 is operating at the preferred stoichiometric ratio maybe achieved in several ways and that, once determined, a feedback loopusing one or more control inputs may be used to control the systemwithin this preferred mode or cause the system to operate in thismanner. One method may be to detect or measure the content of theexhaust gases being emitted from the relevant combustor. This mayinclude sensors 70, such as the sensor for detecting excess oxidant 64,that measures the gases present in the exhaust and/or other relevantcharacteristics. It will be appreciated that a sensor 70 that detectsthe presence of unspent fuel or CO or other gases within the exhaustflow may be used also. Measuring the flow characteristics of the inputs(i.e., the oxidant and the fuel) to one of the combustors also may beused to determine if combustion within the relevant combustor isoccurring at the preferred stoichiometric ratio. In this case, forexample, the oxidant flow into the combustor may be measured, the fuelflow into the combustor may be measured, and a determination made as tothe stoichiometric characteristics of the combustion therein given theseinputs. Other relevant operating characteristics (such as temperature,pressure, etc.) may also be taken into account. Alternatively, or inconjunction with this calculation, unspent fuel or CO or other gasesand/or oxygen may be measured downstream of the combustors or otherpoints within the circulating flow of working fluid. From this, acalculation may be made as to the stoichiometric balance of thecombustion, which may then be compared with the specified set-point orpreferred stoichiometric ratio to determine if it falls within anacceptable range.

At step 106, having already determined that one of the combustors isoperating within the desired stoichiometric range, the logic flowdiagram 100 may determine the current level of extraction at extractionpoint 51. This may be done via checking measured process variables whicheither indicate this flow level directly or may be used to calculate theamount of gas being extracted. The method may further check whether thecurrent level of extraction satisfies a desired level of extraction orspecified set-point. This may be done by comparing the actual level ofextraction (which may be measured) to operator defined set-points orpreference. If it is determined that the desired level of extraction isbeing satisfied, the method may cycle back to step 102 where the processbegins anew. If it is determined that the desired level of extraction isnot being satisfied, the method may proceed to step 108.

At step 108, the method determines one or more “control inputs” that maybe used to manipulate the function of system components in such a way asto achieve the desired level of extraction or, at least, to achieve anextraction level that decreases the difference between the actual levelof extraction and the desired level of extraction. It will beappreciated that a “control input” is one of the many ways by whichoperation of the power plant 9 or any of its components may becontrolled or manipulated. These, for example, may include level of fuelflow to the combustors 22, 24, control of oxidant flow to the combustors22, 24, angle of inlet guide vanes within the compressors 11, 12, etc.Whereas, a “variance amount” is the extent to which a control input mustbe manipulated to bring about the desired manner of operation. Thevariance amount, for example, may include the extent to which the fuelflow to the combustors 11, 12 must be increased or decreased to bringabout desired operation. In certain embodiments, one of the controlinputs that is particularly relevant at step 108 is the setting of theextracted gas valve 61. In this case, the variance amount is the extentto which the setting of the valve 61 needs to be manipulated so that adesired extraction level is achieved. The method may then proceed tostep 110. It will be appreciated that a conventional feedback controlmechanism in conjunction with a PID controller or the like may be usedto achieve control as specified herein. Thus, an iterative process ofvariations to one or more control inputs may bring the system towarddesired operation.

At step 110, in some embodiments, the method may determine the probableeffects to plant operation of each of the available controlinputs/variance amounts from step 108 before making the actual change tothe control input. It will be appreciated that these types ofcalculations may be achieved per conventional power plant controlprograms and modeling software, such as those systems and methodsmention herein and others similar to them. It further will beappreciated that these calculations may involve an iterative processthat takes into account efficient control measures/counter-measureswhich may be made in response to the proposed variance of the relevantcontrol input, economic considerations, wear and tear to the powerplant, operator preferences, plant operational boundaries, etc. Themethod then may proceed to Step 112.

At step 112, the process 100 may determine which of the availablecontrol inputs/variance amounts from the above step is most favorable orpreferred. This determination, in large part, may be based upon theeffects to system operation that were calculated in step 110. Then, forwhichever control input/variance amount is deemed most favorable, themethod may determine if the proposed control input/variance amountshould be executed based on whether the associated benefits of meetingextraction demand outweighs the costs associated with executing thevariance amount. It will be appreciated that economic considerations andoperator preferences may be included in this determination. Based onthis calculation, the method then may execute the proposed controlinput/variance amount or not. The method then may return to step 102,and an iterative process begun by which a preferred level of extractionis achieved.

As described above, if at step 104 it is determined that the relevantcombustor is not operating at the stoichiometric point, the method mayproceed to step 114. At step 114, the method may determine one or morecontrol inputs/variance amounts that are available for achievingstoichiometric point operation within the relevant combustor. As before,control inputs include ways in which operation of the power plant 9 maybe altered, manipulated or controlled, and the variance amount is theextent to which a control input must be manipulated to achieve a desiredmode of operation. The method may then proceed to step 116.

At step 116, the method may determine the probable effects to plantoperation of each of the available control inputs/variance amounts fromstep 114. It will be appreciated that these types of calculations may beachieved per conventional power plant control programs and modelingsoftware, such as those systems and methods mention herein and otherssimilar to them. It further will be appreciated that these calculationsmay involve an iterative process that takes into account efficientcontrol measures/counter-measures which may be made in response to theproposed variance of the relevant control input, economicconsiderations, wear and tear to the power plant, operator preferences,plant operational boundaries, etc. The method then may proceed to Step118.

Plant operation boundaries may include any prescribed limit that must befollowed so that efficient operation is achieved and/or undue wear andtear or more serious damage to systems is avoided. For example,operational boundaries may include maximum allowable temperatures withinthe turbines 30, 32 or combustor components. It will be appreciated thatexceeding these temperatures may cause damage to turbine components orcause increased emissions levels. Another operational boundary includesa maximum compressor pressure ratio across each of the oxidantcompressor 11 and the recirculation compressor 12. Exceeding thislimitation may cause the unit to surge, which may cause extensive damageto components. Further, the turbine may have a maximum mach number,which indicates the maximum flow rate of the combusted gases at the exitof the turbine. Exceeding this maximum flow rate may damage turbinecomponents. Given the possible configuration of combustors within thepower plant 9, relative pressures of the flows delivered at thecombustors 22, 24 by each of the compressors 11, 12 may be anotheroperational boundary. That is, depending on the configuration of thecombustor 22, 24 and the manner in which flows are combined, thepressure of the compressed oxidant delivered by the oxidant compressor11 must be within a certain range of that supplied by recirculationcompressor 12 to avoid aerodynamic losses, backflow, and other potentialissues.

At step 118, the method may determine which of the available controlinputs/variance amounts from the above step is most favorable orpreferred. This determination, in large part, may be based upon theeffects to system operation that were calculated in step 116 as well asthe extent to which the control input/variance amount is able tomanipulate the power plant system toward the intended mode of operation.Then, for whichever control input/variance amount is deemed mostfavorable, the method may determine if the proposed controlinput/variance amount should be executed based on whether the associatedbenefits of achieving stoichiometric point operation (which may includethe benefits of being able to extract working fluid) outweighs the costsassociated with executing the variance amount. It will be appreciatedthat economic considerations and operator preferences may be included inthis determination. Based on this calculation, the method then mayexecute the proposed control input/variance amount or not. The methodthen may return to step 104, and an iterative process by whichstoichiometric point operation within one of the combustors isultimately achieved or determined not possible due to some operationalconstraint.

It will be appreciated that there are many possible controlinputs/variance amounts that affect the stoichiometric ratio in thecombustors 22, 24. In preferred embodiments, one such control inputincludes controllably varying the compressed oxidant amount delivered tothe combustors 22, 24. It will be appreciated that controllably varyingthe supply of compressed oxidant may have a significant effect on thestoichiometry ratio within the combustors 22, 24. For example, ifsensors indicate that, given the supply of fuel to a combustor, morecompressed oxidant (i.e., more oxygen) is needed to achievestoichiometric combustion, the supply of compressed oxidant may beincreased by manipulating inlet guide vanes of the oxidant compressor 11and/or changing valve settings on the oxidant valves 54, 68 so that morecompressed oxidant is able to pass through the oxidant conduit 52, 67associated with the combustor. On the other hand, varying fuel supply isanother control input that may be used to achieve operation at apreferred stoichiometric ratio. In this case, for example, sensors 70may indicate that, given the compressed oxidant amount being deliveredto the combustor, more fuel is needed to achieve stoichiometric pointoperation. The fuel amount being delivered to the one or both combustors22, 24 may be increased by manipulating one or both of the fuel valves58, 59. Further, it will be appreciated that stoichiometric pointcombustion may be controlled in one of the combustors by changingsettings that are directly related to the other combustor. This isbecause changed settings within one combustor may create excess oxidantor unspent fuel in the recirculation loop 10 that is ultimately ingestedwithin the other combustor, thereby affecting the combustionstoichiometric ratio therein.

In one exemplary mode of control, the fuel/oxidant input into the powerplant 9 may be set such that there is excess oxidant (i.e., astoichiometric ratio greater than 1) in whichever of the combustors 22,24 is meant to operate at the stoichiometric point. Then, the controlprocess may decrease the excess oxidant by small increments within therelevant combustor 22, 24 (either by increasing fuel flow to thecombustor or by decreasing the oxidant supply) while monitoring thestoichiometric ratio therein by measuring a relevant process variable.In certain embodiments, this may continue until the stoichiometric ratiois within a preferred range, while still being slightly above 1 (i.e.,still having excess oxidant). This may be implemented by slowlyincreasing oxidant flow, decreasing fuel flow, or both to theparticularly combustor 22, 24, while monitoring stoichiometricconditions therein. It may also be done indirectly by slowly increasingoxidant flow, decreasing fuel flow, or both to the other combustor 22,24 so that excess fuel or oxidant becomes part of the working fluid andingested into the relevant combustor.

FIGS. 8 and 9 provide schematic illustrations of alternativeconfigurations of exemplary power plants according to the presentapplication. As shown, these power plants also employ exhaust gasrecirculation and a reheat combustion system similar to those describedabove. However, the power plants of FIGS. 8 and 9 provide dualextraction locations on the recirculation loop. It will be appreciatedthat while the description of components, system configurations, andcontrol methods provided above are applicable to the power plants ofFIGS. 8 and 9 (as well as some of the functionality described belowbeing applicable to the components, system configuration, and controlmethods described above), the dual extraction locations provide a novelapplication that enables enhanced functionality, which may beneficiallyemployed in certain operating conditions. As before, the power plant 9may include a recirculation loop 10 about which a working fluid isrecirculated. The recirculation loop 10 may include a plurality ofcomponents that are configured to accept an outflow of working fluidfrom a neighboring upstream component and provide an inflow of workingfluid to a neighboring downstream component. The components of therecirculation loop 10 may include: a recirculation compressor 12; anupstream combustor 22 positioned downstream of the recirculationcompressor 12; a high-pressure turbine 30 positioned downstream of theupstream combustor 22; a downstream combustor 24 positioned downstreamof the high-pressure turbine 30; a low-pressure turbine 32 positioneddownstream of the downstream combustor 24; and recirculation conduit 40configured to complete the loop by directing the outflow of workingfluid from the low-pressure turbine 32 to the recirculation compressor12. As described in more detail above in relation to the other exemplarypower plants 9 provided in the several figures, the power plant 9 ofFIGS. 8 and 9 may further include systems and components that controland deliver a compressed oxidant amount to each of the upstreamcombustor and the downstream combustor. As described above in relationto the other exemplary power plants 9, the power plant 9 of FIGS. 8 and9 may further include systems and components that control a fuel amountsupplied to each of the upstream combustor 22 and the downstreamcombustor 24. The power plant 9, as illustrated, may further includesystems and components that extract the working fluid exhausted from theupstream combustor 22 from a first extraction point 75, and systems andcomponents that extract the working fluid exhausted from the downstreamcombustor 24 from a second extraction point 76. The power plant 9, asillustrated in FIGS. 8 and 9 and discussed further above, may includesystems and components for controlling operation such that each of theupstream combustor 22 and the downstream combustor 24 periodicallyoperate at a preferred stoichiometric ratio, as well as means forselectively extracting working fluid from the first extraction point 75and the second extraction point 76 based on which of the upstreamcombustor 22 and the downstream combustor 24 operates at the preferredstoichiometric ratio.

In certain embodiments, the first extraction point 75 may include afirst controllable extracted gas valve 61 for controlling the amount ofgas extracted at that location. The first extraction point 75 may bedisposed on the recirculation loop 10 between the upstream combustor 22and, proceeding in a downstream direction, the downstream combustor 24.As illustrated in FIGS. 8 and 9, one exemplary location for the firstextraction point 75 is the aft end of the high-pressure turbine 30. Thefirst controllable extracted gas valve 61 may be controllable to atleast two settings: a closed setting that prevents the extraction ofworking fluid and an open setting that allows the extraction of workingfluid. Similarly, the second extraction point 76 may include a secondcontrollable extracted gas valve 61 for controlling the amount of gasextracted at that location. The second extraction point 76 may bedisposed on the recirculation loop 10 between the downstream combustor24 and, proceeding in a downstream direction, the upstream combustor 22.As illustrated in FIG. 8, one exemplary location for the secondextraction point 76 is the aft end of the low-pressure turbine 32. Asillustrated in FIG. 9, another exemplary location for the secondextraction point 76 is on the recirculation conduit 40 between thecooler 44 and the blower 46. Depending on the required properties of theextracted gas, other locations are possible. The second controllableextracted gas valve 61 may be controllable to at least two settings: aclosed setting that prevents the extraction of working fluid and an opensetting that allows the extraction of working fluid.

In certain embodiments, the systems and components for controlling thecompressed oxidant amount supplied to the upstream combustor 22 mayinclude an oxidant compressor 11, a first oxidant conduit 52 that isconfigured to direct compressed oxidant derived from the oxidantcompressor 11 to the upstream combustor 22, and a first controllableoxidant valve 54 disposed on the first oxidant conduit 52 that iscontrollable to at least three settings: a closed setting that preventsdelivery of the compressed oxidant to the upstream combustor 22 and twoopen settings that allow delivery of differing compressed oxidantamounts to the upstream combustor 22. In certain embodiments, thesystems and components for controlling the compressed oxidant amountsupplied to the downstream combustor 24 may include the oxidantcompressor 11, a second oxidant conduit 67 that is configured to directcompressed oxidant derived from the oxidant compressor 11 to thedownstream combustor 24, and a second controllable oxidant valve 68disposed on the second oxidant conduit 67 that is controllable to atleast three settings: a closed setting that prevents delivery of thecompressed oxidant to the downstream combustor 24 and two open settingsthat allow delivery of differing compressed oxidant amounts to thedownstream combustor 24. In certain embodiments, a booster compressor 16may be included that is disposed on at least one of the first oxidantconduit 52 and the second oxidant conduit 67 (an example of which isshown in FIG. 6). The booster compressor 16 may be configured to boostthe pressure of the compressed oxidant flowing through at least one ofthe first 52 and the second oxidant conduit 67 such that the compressedoxidant amount supplied to at least one of the upstream 22 and thedownstream combustor 24 comprises a pressure level that corresponds to apreferable injection pressure of whichever of the upstream 22 anddownstream combustor 24. In certain embodiments, at an upstream end, thefirst oxidant conduit 52 may include a first oxidant extraction location81 at which the compressed oxidant is extracted from the oxidantcompressor 11. At an upstream end, the second oxidant conduit 67 mayinclude a second oxidant extraction location 83 at which the compressedoxidant is extracted from the oxidant compressor 11. Within the oxidantcompressor 11, the first oxidant extraction location 81 may include adownstream position relative to the second oxidant extraction location83. The first oxidant extraction location 81 may include a predeterminedposition within the oxidant compressor 11 that corresponds to apreferable injection pressure at the upstream combustor 22. The secondextraction location 83 may include a predetermined position within theoxidant compressor 11 that corresponds to a preferable injectionpressure at the downstream combustor 24.

In certain embodiments, the systems and components for controlling thefuel amount supplied to the upstream combustor 22 may include anupstream combustor fuel supply 78 that may include a controllableupstream combustor fuel valve or first controllable fuel valve 58. Thefirst controllable fuel valve 58 may be controllable to at least threesettings: a closed setting that prevents delivery of fuel to theupstream combustor 22 and two open settings that allow delivery ofdiffering fuel amounts to the upstream combustor 22. The systems andcomponents for controlling the fuel amount supplied to the downstreamcombustor 24 may include a downstream combustor fuel supply 79 that mayinclude a controllable downstream combustor fuel valve or secondcontrollable fuel valve 59. The second controllable fuel valve 59 may becontrollable to at least three settings: a closed setting that preventsdelivery of fuel to the downstream combustor 24 and two open settingsthat allow delivery of differing fuel amounts to the downstreamcombustor 24. In certain embodiments, as shown in FIG. 8, the upstreamcombustor fuel supply 78 and the downstream combustor fuel supply 79 mayhave a common source and, thus, a common fuel type. In otherembodiments, as shown in FIG. 9, the upstream combustor fuel supply 78and the downstream combustor fuel supply 79 may have difference sourcesand may supply differing fuel types.

As describe above in more detail, the power plant 9 of FIGS. 8 and 9 mayinclude systems and components for controlling the power plant 9 suchthat each of the upstream combustor 22 and the downstream combustor 24periodically operate at the preferred stoichiometric ratio. In certainembodiments includes a computerized control unit 65 that is configuredto control the settings of the first and second controllable oxidantvalves 54 and the first and second controllable fuel valves 58, 59.

As described in more detail above, in certain embodiments, the powerplant 9 of FIGS. 8 and 9 may include systems and components fordetermining a current stoichiometric ratio at which the upstreamcombustor 22 and the downstream combustor 24 operate. In certainexemplary embodiments, the systems and components for determining thecurrent stoichiometric ratio at which the upstream combustor 22 and thedownstream combustor 24 operate include: systems and components formeasuring the compressed oxidant amount being supplied to the upstreamand downstream combustors 22, 24 and systems and components formeasuring the fuel amount being supplied to the upstream and downstreamcombustors 22, 24; and systems and components for calculating thecurrent stoichiometric ratio at which each of the upstream combustor 22and the downstream combustor 24 operates based on the measuredcompressed oxidant amounts and the measured fuel amount being suppliedto each. In certain exemplary embodiments, the systems and componentsfor determining the stoichiometric ratio at which the upstream combustor22 and the downstream combustor 24 operate include: a first testingcomponent for testing the working fluid exhausted from the upstreamcombustor 22; and a second testing component for testing the workingfluid exhausted from the downstream combustor 24. The first testingcomponent and the second testing component each may include one of asensor for detecting excess oxidant and a sensor for detecting unspentfuel. One or more CO sensors and one or more humidity sensors may alsobe used, as one of ordinary skill in the art will appreciate. The firsttesting location may include a location within a range of positions onthe recirculation loop 10. The range of positions may be defined betweenthe first extraction point 75 and, proceeding in an upstream direction,the upstream combustor 22. The second testing location may include alocation within a range of positions on the recirculation loop 10. Therange of positions may be defined between the second extraction point 76and, proceeding in an upstream direction, the downstream combustor 24.

In certain embodiments, the systems and components for selectivelyextracting from the first extraction point 75 and the second extractionpoint 76 based on which of the upstream combustor 22 and the downstreamcombustor 24 is being operated at the preferred stoichiometric ratioincludes a computerized control unit 65. In one preferred embodiment,the control unit 65 is configured to: extract working fluid from thefirst extraction point 75 during periods when the upstream combustor 22operates at the preferred stoichiometric ratio; and extract workingfluid from the second extraction point 76 during periods when thedownstream combustor 24 operates at the preferred stoichiometric ratio.

As provided herein, the power plant of FIGS. 8 and 9 may be operated pernovel control methods. In certain embodiments, such methods may includethe steps of: recirculating at least a portion of the working fluidthrough the recirculation loop 10; controlling a compressed oxidantamount supplied to each of the upstream combustor 22 and the downstreamcombustor 24; controlling a fuel amount supplied to each of the upstreamcombustor 22 and the downstream combustor 24; controlling the powerplant 9 such that each of the upstream combustor 22 and the downstreamcombustor 24 periodically operates at a preferred stoichiometric ratio;and selectively extracting the working fluid from a first extractionpoint 75 associated with the upstream combustor 22 and a secondextraction point 76 associated with the downstream combustor 24 basedupon which of the upstream 22 and the downstream combustor 24 operatesat the preferred stoichiometric ratio. The step of selectivelyextracting the working fluid from the first 75 and the second extractionpoints 76 may include selecting to extract from the first extractionpoint 75 only during periods when the upstream combustor 22 operates atthe preferred stoichiometric ratio, and selecting to extract workingfluid from the second extraction point 76 only during periods when thedownstream combustor 24 operates at the preferred stoichiometric ratio.In one preferred embodiment, for example, the upstream combustor 22 maybe operated at the preferred stoichiometric ratio during low-loadoperation, and the downstream combustor 24 may be operated at thepreferred stoichiometric ration during full operation. The step ofselectively extracting working fluid from the first 75 and secondextraction points 76 may include controlling the settings of the first61 and second controllable extracted gas valves 61. The step ofcontrolling the compressed oxidant amount supplied to each of theupstream and downstream combustors 22, 24 may include manipulating thesettings of the first and second controllable oxidant valves 54, 68. Thestep of controlling the fuel amounts supplied to each of the upstreamand downstream combustors 22, 24 may include the steps of manipulatingthe settings of the first and second controllable fuel valves 58, 59.

The step of controlling the power plant 9 such that each of the upstreamcombustor 22 and the downstream combustor 24 periodically operate at thepreferred stoichiometric ratio may include using a computerized controlunit 65 that is configured to control the settings of the first andsecond controllable oxidant valves 54 and the first 58 and secondcontrollable fuel values 59. The preferred stoichiometric ratio mayinclude a stoichiometric ratio of about 1, though the other rangesdiscussed herein are also possible.

In certain embodiments, the method may include the steps of: measuring aplurality of process variables of the power plant 9; determining anoutput requirement for the power plant 9; based on the measured processvariables and the output requirement, determining a desired mode ofoperation for the power plant 9; determining a preferred stoichiometriccombustor, the preferred stoichiometric combustor including whichever ofthe upstream combustor 22 and the downstream combustor 24 is preferredfor operation at the preferred stoichiometric ratio given the desiredmode of operation for the power plant 9 and a chosen criteria; andcontrolling the power plant 9 such that the preferred stoichiometriccombustor operates at the preferred stoichiometric ratio. It will beappreciated that power plants configured as with dual combustion systemsmay choose to shut-down one of the combustion systems during a turndownmode of operation, thereby more efficiently satisfying a lower outputrequirement. Accordingly, in certain embodiments, the desired mode ofoperation includes a turndown mode of operation during which only one ofthe upstream combustor 22 and the downstream combustor 24 operates. Inthis case, the preferred stoichiometric combustor may include whicheverof the upstream combustor 22 and the downstream combustor 24 operatesduring the turndown mode of operation. In certain embodiments, theupstream combustor 22 is the combustor that operates during the turndownmode of operation.

The chosen criteria for determining the preferred stoichiometriccombustor may be any of several. In certain preferred embodiments, thechosen criteria relates to the efficiency level of the power plant 9. Inthis manner, the preferred stoichiometric combustor is the combustorthat, when operated at the preferred stoichiometric ratio, promotesefficiency. The chosen criteria also may be related to economicconsiderations, i.e., the preferred stoichiometric combustor is the onethat promotes the profits of the power plant 9.

In certain embodiments, the method of the present application mayfurther include the steps of: determining a current stoichiometric ratioat which the preferred stoichiometric combustor operates; determiningwhether the current stoichiometric ratio is equal to the preferredstoichiometric ratio; and extracting working fluid from the extractionpoint associated with the preferred stoichiometric combustor if thecurrent stoichiometric ratio is determined to be equal to the preferredstoichiometric ratio. In certain embodiments, this may include the stepsof: measuring the compressed oxidant amount being supplied to theupstream and downstream combustors 22, 24; measuring the fuel amountbeing supplied to the upstream and downstream combustors 22, 24; andcalculating the current stoichiometric ratio at which the preferredstoichiometric combustor operates based on the measured compressedoxidant amount being supplied to the upstream and downstream combustors22, 24 and the measured fuel amounts being supplied to the upstream anddownstream combustors 22, 24. In certain embodiments, the step ofdetermining the current stoichiometric ratio at which the preferredstoichiometric combustor operates includes the steps of: if the upstreamcombustor 22 may include the preferred stoichiometric combustor, testingthe working fluid exhausted from the upstream combustor 22; and if thedownstream combustor 24 may include the preferred stoichiometriccombustor, testing the working fluid exhausted from the downstreamcombustor 24. The working fluid exhausted from the upstream combustor 22may be tested at a first test location by one of a sensor for detectingexcess oxidant and a sensor for detecting unspent fuel. The first testlocation may include a location within a range of locations on therecirculation loop defined between the first extraction point 75 and,proceeding in an upstream direction, the upstream combustor 22. Theworking fluid exhausted from the downstream combustor 24 may be testedat a second test location by one of a sensor for detecting excessoxidant and a sensor for detecting unspent fuel. The second testlocation may include a location within a range of locations on therecirculation loop defined between the second extraction point 76 and,proceeding in an upstream direction, the downstream combustor 24. Inthis manner, the status of the exhaust prior to extraction may be testedto confirm desired properties.

In certain embodiments, the step of controlling the power plant 9 suchthat the preferred stoichiometric combustor operates at the preferredstoichiometric ratio includes the step of operating a feedback loopcontrol mechanism that includes manipulating a control input of thepower plant 9 based on the measured plurality of the process variables.The methods of operating a feedback loop control mechanism are discussedin more detail above. In certain cases, it will be appreciated that thestep of measuring the plurality of process variables may includemeasuring the compressed oxidant amount and the fuel amount supplied tothe preferred stoichiometric combustor and calculating a currentstoichiometric ratio in the preferred stoichiometric combustor based onthe measured compressed oxidant amount and fuel amount supplied to thepreferred stoichiometric combustor. In certain embodiments, the controlinput may include the settings for whichever of the first and secondcontrollable oxidant valves 54, 68 correspond to the preferredstoichiometric combustor and whichever of the first and secondcontrollable fuel valves 58, 59 correspond to the preferredstoichiometric combustor.

In certain embodiments, the step of measuring the plurality of processvariables may include measuring the compressed oxidant amounts and thecompressed fuel amounts being supplied to each of the upstream anddownstream combustors 22, 24. The step of calculating the currentstoichiometric ratio in the preferred stoichiometric combustor mayinclude balancing, in each of the upstream and downstream combustors 22,24, the measured oxygen amount against the measured fuel amount todetermine whether the preferred stoichiometric combustor ingests anexcess fuel amount or an excess oxidant amount that is present in theworking fluid from whichever of the upstream and downstream combustors22, 24 is not the preferred stoichiometric combustor.

In certain embodiments, the step of measuring the plurality of processvariables may include testing a working fluid content at a position onthe recirculation loop that is both downstream of the preferredstoichiometric combustor and upstream of whichever of the upstream anddownstream combustors 22, 24 is not the preferred stoichiometriccombustor. The control input may include at least one of the fuel amountsupplied to the upstream combustor 22, the fuel amount supplied to thedownstream combustor 24, the compressed oxidant amount supplied to theupstream combustor 22, and the compressed oxidant amount supplied to thedownstream combustor 24. The step of testing the working fluid contentmay include measuring at least one of an oxidant content and an unspentfuel content of the working fluid, which may further include the step ofcalculating a current stoichiometric ratio in the preferredstoichiometric combustor based on the testing of the working fluidcontent.

In certain exemplary embodiments, the method of the present applicationincludes controlling the power plant 9 such that both of the upstreamcombustor 22 and the downstream combustor 24 periodically operate at thepreferred stoichiometric ratio during the same period of time. In thiscase, selectively extracting the working fluid from the first extractionpoint 75 and the second extraction point 76 may include extractingworking fluid from both the first extraction point 75 and the secondextraction point 76 when both combustors 22, 24 operate at the preferredstoichiometric ratio. As shown in FIGS. 8 and 9, the two extracted gasflows may be combined at a combining point 86. That is, the method ofthe present application may include the step of combining the workingfluid extracted from the first extraction point 75 and the working fluidextracted from the second extraction point 76. The method may furtherinclude the step of controllably combining the two extracted flows ofworking fluid such that a combined flow of extracted working fluidincludes a desired characteristic. It will be appreciated that this maybe done by controlling the settings of the controllable extracted gasvalves 61 that are included at each extraction point 75, 76. Dependingon the downstream applications for which the extracted gas is extracted,it is beneficial to have the ability to provide the extracted gas atvarying pressure levels or temperatures. This may be achieved by mixingthe gases extracted from different points on the recirculation loop 10in desired or controlled amounts. As shown in FIG. 9, the firstextraction point 75 extracts gas from a region of relative hightemperature and high pressure, while the second extraction point 76extracts gas from a region of relatively low temperature and lowpressure. It will be appreciated that by mixing the two flows in acontrolled manner, desired extracted gas characteristics within therange of characteristics defined by the differing extraction locationmay be achieved.

Turning now to FIGS. 10-13, schematic drawings illustratingconfigurations of alternative power plants that employ exhaust gasrecirculation and a single combustion system are provided. It will beappreciated that the power plant 9 of these figures includes many of thesame components as the power plants described above and that thesecomponents may be employed in much the same manner at that describedelsewhere in this application. As stated, any of the descriptionspertaining to any of the power plants that one of ordinary skill in theart would appreciate as not being limited to a specific configuration isapplicable to all the configurations, particularly as such alternativesmay be described in the claims or any amendments made thereto. Incertain embodiments, the power plant 9 is configured to include arecirculation loop 10 about which a working fluid is recirculated. Asbefore, the recirculation loop 10 may include a plurality of componentsconfigured to accept an outflow of working fluid from a neighboringupstream component and provide an inflow of working fluid to aneighboring downstream component. In this case, the recirculation loop10 includes a recirculation compressor 12; a combustor 22 positioneddownstream of the recirculation compressor 12; a turbine 30 positioneddownstream of the combustor 22; and a recirculation conduit 40configured to direct the outflow of working fluid from the turbine 30 tothe recirculation compressor 12. The power plant 9 is configured to havea single combustion system. As such, the recirculation loop 10 may beconfigured to prevent the input of combustion gases at all locationsexcept for an input related to the combustor 22. As shown, the powerplant 9 may further include a first extraction point 75 and a secondextraction point 76 positioned on the recirculation loop 10. The outflowof working fluid from the turbine 30 includes exhaust gases, which, viathe recirculation conduit 40, are directed to the recirculationcompressor 12. The recirculation compressor 12 is configured to compressthe exhaust gases such that the outflow of working fluid from therecirculation compressor 12 includes compressed exhaust gases;

The first extraction point 75 may include a controllable extractionvalve 61 that is controllable to at least two settings: a closed settingthat prevents the extraction of working fluid and an open setting thatallows the extraction of working fluid. The second extraction point 76may include a controllable extraction valve 61 that is controllable toat least two settings: a closed setting that prevents the extraction ofworking fluid and an open setting that allows the extraction of workingfluid.

The power plant 9 may be operated or controlled such that the combustor22 at least periodically operates at a preferred stoichiometric ratio.The preferred stoichiometric ratios may be similar to those ratiosdiscussed above. To achieve this type of operation, a compressed oxidantamount and a fuel amount supplied the combustor 22 may be controlled.The compressed oxidant amount may be controlled by an oxidant compressor11, an oxidant conduit 52 that is configured to direct compressedoxidant derived from the oxidant compressor 11 to the combustor 22, anda controllable oxidant valve 54 disposed on the oxidant conduit that iscontrollable to at least two open settings that allow delivery ofdiffering compressed oxidant amounts to the combustor 22. The fuelamount may be controlled by a controllable fuel valve 58 that has atleast two open settings that allow delivery of differing fuel amounts tothe combustor 22. It will be appreciated that the power plant 9 may becontrolled such that the combustor 22 at least periodically operates atthe preferred stoichiometric ratio via a computerized control unit 65that is configured to control the settings of the controllable oxidantvalve 54 and the controllable fuel valve 58, and may include systems fordetermining a current stoichiometric ratio at which the combustor 22operates, the various systems for which are discussed in detail above,whether the current stoichiometric ratio is equal to the preferredstoichiometric ratio, as well as a control feedback loop mechanism thatachieves the desired modes of operations. As discussed in more detailbelow, the computerized control unit 65 may be configured to selectivelyextract working fluid from at least one of the first extraction point 75and the second extraction point 76 based on whether the currentstoichiometric ratio in the combustor 22 is determined to be equal tothe preferred stoichiometric ratio, as well as the intended downstreamuses of the extracted working fluid.

In certain embodiments, the power plant 9 includes a recirculationconduit 40 that is configured to collect exhaust gases from the turbine30 and direct the exhaust gases to an intake of the recirculationcompressor 12. The recirculation conduit 40 may further include a heatrecovery steam generator, the heat recovery steam generator including aboiler, the heat recovery steam generator being configured such that theexhaust gases from the turbine 30 includes a heat source for the boiler.The recirculation conduit 40 may include a chiller 44 and a blower 46positioned thereon. The chiller 44 may be configured to controllablyremove an amount of heat from the exhaust gases flowing through therecirculation conduit 40 such that a more desirable temperature isachieved at the intake of the recirculation compressor 12. The blower 46may be configured to controllably circulate the exhaust gases flowingthrough the recirculation conduit 40 such that a more desirable pressureis achieved at the intake of the recirculation compressor 12.

The power plant 9 may include instruments, sensors, and systems fordetermining a property of characteristic of the working fluid at theextraction points 75, 76. These may include direct measurement of thecharacteristic or calculation based on other measured process variables.The characteristic may include any property of the working fluid, suchas, pressure and temperature. As stated, the extracted working fluid haseconomic value in certain industrial and other applications. It will beappreciated that if the extracted working fluid may be efficientlydelivered with desired characteristics given an intended application,such as at a desired pressure or temperature, the value of it isincreased. In certain embodiments, the means for determining thecharacteristic of the working fluid at the first extraction point 75 andthe second extraction point 76 may include a pressure sensor and/or atemperature sensor. The computerized control unit 65 may be configuredto selectively extract the working fluid from only or just the firstextraction point 75, just the second extraction point 76, or both thefirst and second extraction point 75, 76 based on the characteristic ofthe working fluid that is determined to be at each of the extractionpoints 75, 76. The computerized control unit 65 may do this viacontrolling the settings of the first and second controllable extractionvalves 61.

The computerized control unit 65 may be configured to determine apreferred value for the characteristic of the working fluid. This may bedone via determining an intended downstream application for theextracted working fluid, which could be completed via consulting anoperator entered value or otherwise. The system then could determine apreferred value for the characteristic of the working fluid based onwhat would be a preferred value given the intended downstreamapplication.

The extraction points 75, 76 may include various locations. While a fewpreferred embodiments relating to extraction point configuration areprovided in FIGS. 10-13, it will be appreciated that others arepossible. As shown in FIG. 10, the first extraction point 75 may have alocation within the recirculation compressor 12, and the secondextraction point 76 may have a location within the turbine 30. As shownin FIG. 11, the first extraction point 75 may have a location within therecirculation compressor 12, and the second extraction point 76 may havea location within the recirculation conduit 40. As shown in FIG. 12, thefirst extraction point 75 may have a first location within therecirculation compressor 12, and the second extraction point 76 may havea second location within the recirculation compressor 12. As shown inFIG. 13, the first extraction point 75 may have a first location withinthe turbine 30, and the second extraction point 76 may have a secondlocation within the turbine 30. The advantages of these configurationsare discussed in more detail below.

The present application further describes a method of controlling apower plant that includes configurations discussed above in relation toFIGS. 10-13. In general, these methods may include the steps of:recirculating at least a portion of the working fluid through therecirculation loop; controlling the power plant such that the combustor22 at least periodically operates at a preferred stoichiometric ratio;and extracting working fluid from at least one of a first extractionpoint 75 and a second extraction point 76 positioned on therecirculation loop 10 during the periods when the combustor 22 operatesat the preferred stoichiometric ratio. The step of controlling the powerplant such that the combustor 22 periodically operates at the preferredstoichiometric ratio may include the steps of controlling a compressedoxidant amount and a fuel amount supplied to the combustor 22.

The method may further include the steps of: determining acharacteristic of the working fluid at the first extraction point 75;determining a characteristic of the working fluid at the secondextraction point 76; and, based on the characteristic of the workingfluid at the first and second extraction points 75,76 selectivelyextracting the working fluid from just the first extraction point 75,just the second extraction point 76, or both the first and secondextraction points 75, 76. Based on a downstream application, the methodmay determine a preferred value for the characteristic of the workingfluid, which may also be used to selectively extract working fluid fromthe extraction points 75, 76. This type of method of operation mayresult in working fluid being extracted from both the first extractionpoint 75 and the second extraction point 76 at the same time. In thisinstance, the method may controllably mix the extracted flows of workingfluid from both extraction points 75, 76 so as to create a combined flowof extracted working fluid that has a characteristic consistent with thepreferred value for the characteristic. As before, the preferred valuefor the characteristic of the working fluid may be based on an intendeddownstream application. A computerized control unit 65 may be configuredto control the settings of the various valves and other componentsdiscussed herein so that the desired modes of operation are achieved.

In certain embodiments, the step of selectively extracting the workingfluid from just the first extraction point 75, just the secondextraction point 76, or both the first and second extraction points 75,76 includes the steps of: when the characteristic of the working fluidat the first extraction point 75 is within a predetermined rangerelative to the preferred value for the characteristic, extracting fromjust the first extraction point 75; when the characteristic of theworking fluid at the second extraction point 76 is within apredetermined range relative to the preferred value for thecharacteristic, extracting from just the second extraction point 76;when the preferred value for the characteristic is within apredetermined range nested between the characteristic of the workingfluid at the first extraction point 75 and the characteristic of theworking fluid at the second extraction point 76, and extracting fromboth the first and second extraction points 75, 76. In this manner, themethod may employ just one extraction point when the desiredcharacteristic may be achieved this way, or extract from both extractionpoints when mixing may be employed to deliver the extracted gases in amore desirable state given a downstream application. In certainembodiments, these steps may include the following: when thecharacteristic of the working fluid at the first extraction point 75 isapproximately equal to the preferred value for the characteristic,extracting from the first extraction point 75; when the characteristicof the working fluid at the second extraction point 76 is approximatelyequal to the preferred value for the characteristic, extracting from thesecond extraction point 76; and when the preferred value for thecharacteristic falls in between the characteristic of the working fluidat the first extraction point 75 and the characteristic of the workingfluid at the second extraction point 76, extracting from both the firstand second extraction points 75, 76. When the method operates to extractworking fluid from both extraction points 75, 76, a mixing step, asmentioned, may be employed to create a combined flow that is moredesirable. In certain embodiments, this may be achieved by controllingthe setting of the first controllable extraction valve 61 such that afirst predetermined amount of working fluid is extracted from the firstextraction point 75; controlling the setting of the second controllableextraction valve 61 such that a second predetermined amount of workingfluid is extracted from the second extraction point 76; and combiningthe first predetermined amount of working fluid with the secondpredetermined amount of working fluid at a combining junction such thatthe combined flow of extracted working fluid is formed. It will beappreciated that, given the characteristic of the working fluid at thefirst extraction point 75 and the second extraction point 76, the firstpredetermined amount of working fluid extracted from the firstextraction point 75 and the second predetermined amount of working fluidextracted from the second extraction point 76 may include predeterminedamounts of working fluid that, once mixed, result in the combined flowof extracted working fluid having the preferred value for thecharacteristic. As stated, the characteristic may be one of pressure andtemperature, though others are possible.

The extraction point locations may be predetermined to provide desiredoperation, efficiency, and flexibility in delivering extracted flowshaving desired characteristics. Generally, the first extraction point 75may have a predetermined first location within the recirculation loop 10and the second extraction point 76 may have a predetermined secondlocation within the recirculation loop 10. In one preferred embodiments,the first predetermined location within the recirculation loop 10 andthe second predetermined location within the recirculation loop 10 areselected such that the working fluid at each include a dissimilar firstcharacteristic and a similar second characteristic. In this case, theworking fluid extracted from the first extraction point 75 and thesecond extraction point 76 may be mixed to achieve a wide range oflevels for the first characteristic, while the mixing has little effecton the resulting second characteristic, which will remain at about thelevel of the similar second characteristics of the extracted flows. Inother cases, the first predetermined location within the recirculationloop 10 and the second predetermined location within the recirculationloop 10 may be selected such that the working fluid at each includes adissimilar first characteristic and a dissimilar second characteristic.This time, the working fluid extracted from the first extraction point75 and the second extraction point 76 may be mixed to achieve a widerange of first characteristic values and a wide range of secondcharacteristic values.

Referring to FIG. 10, it will be appreciated that the position withinthe recirculation compressor 12 for the first extraction point 75 andthe location within the turbine 30 for the second extraction point 76may be selected such that the dissimilar first characteristic ispressure and the similar second characteristic is temperature. Referringto FIG. 11, it will be appreciated that the position within therecirculation compressor 12 for the first extraction point 75 and thelocation within the recirculation conduit 40 for the second extractionpoint 76 may be selected such that the dissimilar first characteristicis pressure and the similar second characteristic is temperature. Thelocation for the second extraction point 76 may be varied to produceother results, such as producing a dissimilar temperaturecharacteristic. Another possible configuration includes positioningfirst extraction point 75 in the turbine 30 and the second extractionpoint 76 in the recirculation conduit 40 so that dissimilar pressure anddissimilar temperature characteristics at the two extraction locationsare achieved. It will be appreciated that this type of arrangement mayprovide great flexibility in the mixing of extracted flows to achieve abroad range of values for each of the pressure and temperaturescharacteristics.

In another embodiment, as illustrated in FIG. 12, the first extractionpoint 75 may have a first predetermined location within therecirculation compressor 12, which may be selected to provide a desiredpressure or temperature level for the extracted working fluid during ananticipated first mode of operation for the power plant 9. The secondextraction point 76 may have a second predetermined location within therecirculation compressor 12, which may be selected to provide thedesired pressure or temperature level for extracted working fluid duringan anticipated second mode of operation for the power plant 9. It willbe appreciated that this configuration provides the flexibility ofextracting working fluid at a consistent pressure or temperature level,i.e., the desired pressure or temperature level, no matter if the powerplant 9 is operating in the first or second mode of operation. In apreferred embodiment, the modes coincide with a base load mode ofoperation and a turndown mode of operation. It will be appreciated thatthis configuration further provides the advantageous alternative ofextracting at different pressure or temperature levels during thosetimes when the operation mode of the power plant 9 remains unchanged.

In another embodiment, as illustrated in FIG. 13, the first extractionpoint 75 may have a first predetermined location within the turbine 30,which may be selected to provide a desired pressure or temperature levelfor extracted working fluid during an anticipated first mode ofoperation for the power plant 9. The second extraction point 76 may havea second predetermined location within the turbine 30, which may beselected to provide the desired pressure or temperature level forextracted working fluid during an anticipated second mode of operationfor the power plant 9. In this case, it will be appreciated that theconfiguration provides the flexibility of extracting working fluid at aconsistent pressure or temperature level, i.e., the desired pressure ortemperature level, no matter if the power plant 9 is operating in thefirst or second mode of operation. In a preferred embodiment, the modescoincide with a base load mode of operation and a turndown mode ofoperation. It will be appreciated that this configuration furtherprovides the advantageous alternative of extracting at differentpressure or temperature levels during those times when the operationmode of the power plant 9 remains unchanged.

From the above description of preferred embodiments of the invention,those skilled in the art will perceive improvements, changes andmodifications. Such improvements, changes and modifications within theskill of the art are intended to be covered by the appended claims.Further, it should be apparent that the foregoing relates only to thedescribed embodiments of the present application and that numerouschanges and modifications may be made herein without departing from thespirit and scope of the application as defined by the following claimsand the equivalents thereof.

We claim:
 1. A power plant configured to include a recirculation loopabout which a working fluid is recirculated, the recirculation loopcomprising a plurality of components configured to accept an outflow ofworking fluid from a neighboring upstream component and provide aninflow of working fluid to a neighboring downstream component, whereinthe recirculation loop includes: a recirculation compressor; an upstreamcombustor positioned downstream of the recirculation compressor; ahigh-pressure turbine positioned downstream of the upstream combustorand drivingly connected to the recirculation compressor; a downstreamcombustor positioned downstream of the high-pressure turbine; a lowpressure turbine positioned downstream of the downstream combustor; anda recirculation conduit connected to an inlet of the recirculationcompressor configured to direct the outflow of working fluid from thelow-pressure turbine to the recirculation compressor, the power plantcomprising: an oxidant compressor configured to provide compressedoxidant to both the upstream combustor and the downstream combustor; andmeans for extracting a portion of the working fluid from an extractionpoint disposed at a predetermined location on the recirculation loop. 2.The power plant according to claim 1, wherein: the outflow of workingfluid from the low-pressure turbine comprises exhaust gases, which, viathe recirculation conduit, are directed to the recirculation compressor;the recirculation compressor is configured to compress the exhaust gasessuch that the outflow of working fluid from the recirculation compressorcomprises compressed exhaust gases; and the means for extracting theportion of the working fluid from the extraction point includes meansfor controlling an extracted working fluid amount that is extracted atthe extraction point.
 3. The power plant according to claim 2, whereinthe recirculation conduit is configured to collect the portion of theexhaust gases from the low-pressure turbine and direct the portion ofthe exhaust gases through a stretch of conduit such that the exhaustgases are delivered to an intake of the recirculation compressor;wherein the recirculation conduit further comprises a heat recoverysteam generator, the heat recovery steam generator including a boiler;and wherein the heat recovery steam generator is configured such thatthe exhaust gases from the low-pressure turbine comprises a heat sourcefor the boiler.
 4. The power plant according to claim 2, wherein atleast one of a chiller and a blower are positioned on the recirculationconduit; wherein the chiller comprises means for controllably removingan amount of heat from the exhaust gases flowing through therecirculation conduit such that a more desirable temperature is achievedat the intake of the recirculation compressor; and wherein the blowercomprises means for controllably circulating the exhaust gases flowingthrough the recirculation conduit such that a more desirable pressure isachieved at the intake of the recirculation compressor.
 5. The powerplant according to claim 2, further comprising at least one of anupstream combustor fuel supply and a downstream combustor fuel supply;wherein: when present, the upstream combustor fuel supply includes meansfor controllably varying a fuel amount supplied to the upstreamcombustor; and when present, the downstream combustor fuel supply thatincludes means for controllably varying a fuel amount supplied to thedownstream combustor.
 6. The power plant according to claim 5, furthercomprising: a first oxidant conduit configured to channel compressedoxidant from the oxidant compressor to the upstream combustor, the firstoxidant conduit comprising means for controllably varying a compressedoxidant amount supplied to the upstream combustor; and a second oxidantconduit configured to channel compressed oxidant to the downstreamcombustor, the second oxidant conduit comprising means for controllablyvarying a compressed oxidant amount supplied to the downstreamcombustor.
 7. The power plant according to claim 6, further comprising abooster compressor disposed on at least one of the first oxidant conduitand the second oxidant conduit; and wherein the booster compressor isconfigured to boost the pressure of the compressed oxidant flowingthrough at least one of the first the oxidant conduit and the secondoxidant conduit such that the compressed oxidant supplied to theupstream combustor or downstream combustor comprises a pressure levelthat corresponds to a preferable injection pressure at the upstream ordownstream combustor.
 8. The power plant according to claim 6, wherein,at an upstream end, the second oxidant conduit comprises a connectionwith the first oxidant conduit.
 9. The power plant according to claim 8,further comprising an atmosphere vent disposed on the first oxidantconduit between the oxidant compressor and the booster compressor, theatmosphere vent configured to controllably vary a compressed oxidantamount vented to the atmosphere.
 10. The power plant according to claim9, wherein the first oxidant conduit comprises a booster compressor, thebooster compressor configured to boost the pressure of the compressedoxidant flowing through the first oxidant conduit; and wherein theconnection made by the second oxidant conduit to the first oxidantconduit comprises a downstream position relative to the boostercompressor.
 11. The power plant according to claim 9, wherein the firstoxidant conduit comprises a booster compressor, the booster compressorbeing configured to boost the pressure of the compressed oxidant flowingthrough the first oxidant conduit; and wherein the connection made bythe second oxidant conduit to the first oxidant conduit comprises anupstream position in relative to the booster compressor.
 12. The powerplant according to claim 6, wherein: at an upstream end, the firstoxidant conduit comprises a first oxidant extraction location at whichthe compressed oxidant is extracted from the oxidant compressor; at anupstream end, the second oxidant conduit comprises a second oxidantextraction location at which the compressed oxidant is extracted fromthe oxidant compressor; and within the oxidant compressor, the firstoxidant extraction location comprises a downstream position relative tothe second oxidant extraction location.
 13. The power plant according toclaim 12, wherein the first oxidant extraction location comprises aposition within a compressor discharge casing of the oxidant compressor,and wherein the second oxidant extraction location comprises a stageupstream of the compressor discharge casing within the oxidantcompressor.
 14. The power plant according to claim 12, wherein the firstoxidant extraction location comprises a predetermined position withinthe oxidant compressor based upon a preferable injection pressure at theupstream combustor; and wherein the second extraction location comprisesa predetermined position within the oxidant compressor based upon apreferable injection pressure at the downstream combustor.
 15. The powerplant according to claim 6, further comprising means for controlling thepower plant such that one of the upstream combustor and the downstreamcombustor operates at a preferred stoichiometric ratio; wherein thepredetermined location of the extraction point comprises a range ofpositions on the recirculation loop, the range of positions beingdefined between whichever of the upstream combustor and the downstreamcombustor is operable at the preferred stoichiometric ratio and,proceeding in a downstream direction, the other of the upstream anddownstream combustors.
 16. The power plant according to claim 15,wherein the means for controlling the power plant one of the upstreamcombustor and the downstream combustor at the preferred stoichiometricratio includes a computerized control unit that is configured to controlthe operation of the following components: the means for controllablyvarying the compressed oxidant amount supplied to the upstreamcombustor; the means for controllably varying the compressed oxidantamount supplied to the downstream combustor; the means for controllablyvarying the fuel amount supplied to the upstream combustor; and themeans for controllably varying the fuel amount supplied to thedownstream combustor; and wherein the preferred stoichiometric ratiocomprises a stoichiometric ratio near
 1. 17. The power plant accordingto claim 16, wherein the preferred stoichiometric ratio comprises astoichiometric ratio of between 0.75 and 1.25.
 18. The power plantaccording to claim 16, wherein the preferred stoichiometric ratiocomprises a stoichiometric ratio of between 0.9 and 1.1.
 19. The powerplant according to claim 16, wherein the preferred stoichiometric ratiocomprises a stoichiometric ratio of between 1.0 and 1.1.
 20. The powerplant according to claim 15, wherein the at least one of the upstreamcombustor fuel supply and the downstream combustor fuel supply comprisesthe upstream combustor fuel supply and not the downstream fuel supply;and wherein the means for controlling the power plant such that one ofthe upstream combustor and the downstream combustor operates at thepreferred stoichiometric ratio comprises means for controlling the powerplant such that the downstream combustor operates at the preferredstoichiometric ratio.
 21. The power plant according to claim 20, whereinthe predetermined location of the extraction point comprises a range ofpositions on the recirculation loop, the range of positions beingdefined between the downstream combustor and, proceeding in a downstreamdirection, the upstream combustor.
 22. The power plant according toclaim 21, wherein: the upstream combustor is configured to combine thecompressed oxidant from the oxidant compressor with the compressedexhaust gases from the recirculation compressor and, there within,combust the fuel from the upstream combustor fuel supply; and thedownstream combustor is configured to combine the compressed oxidantfrom the oxidant compressor with the exhaust gases from thehigh-pressure turbine and, there within, combust an excess fuelcontained in the exhaust gases received from the high-pressure turbine.23. The power plant according to claim 21, further comprising means fortesting the working fluid to determine whether the downstream combustoris operating at the preferred stoichiometric ratio; wherein the meansfor testing the working fluid is positioned on the recirculation looprelative to the predetermined position of the extraction point.
 24. Thepower plant according to claim 23, the means for testing the workingfluid comprises at least one of a sensor for detecting excess oxidantand a sensor for detecting unspent fuel; and wherein the position of themeans for testing the working fluid on the recirculation loop comprisesa range of positions, the range of positions being defined between theextraction point and, proceeding in an upstream direction, thedownstream combustor.
 25. The power plant according to claim 15, whereinthe at least one of the upstream combustor fuel supply and thedownstream combustor fuel supply comprises the downstream combustor fuelsupply and not the upstream combustor fuel supply; wherein the means forcontrolling the power plant such that one of the upstream combustor andthe downstream combustor operates at the preferred stoichiometric ratiocomprises means for controlling the power plant such that the upstreamcombustor operates at the preferred stoichiometric ratio; and whereinthe predetermined location of the extraction point comprises a range ofpositions on the recirculation loop, the range of positions beingdefined between the upstream combustor and, proceeding in a downstreamdirection, the downstream combustor.
 26. The power plant according toclaim 25, further comprising means for testing the working fluid todetermine whether the downstream combustor is operating at the preferredstoichiometric ratio; wherein the means for testing the working fluidcomprises a sensor for detecting excess oxidant; and wherein theposition of the means for testing the working fluid on the recirculationloop comprises a range of positions, the range of positions beingdefined between the extraction point and, proceeding in an upstreamdirection, the upstream combustor.
 27. The power plant according toclaim 15, wherein the at least one of the upstream combustor fuel supplyand the downstream combustor fuel supply comprises both of the upstreamcombustor fuel supply and the downstream combustor fuel supply; andwherein the means for controlling the power plant such that one of theupstream combustor and the downstream combustor operates at thepreferred stoichiometric ratio comprises means for controlling the powerplant such that the downstream combustor operates at the preferredstoichiometric ratio.
 28. The power plant according to claim 27, whereinthe predetermined location of the extraction point comprises a range ofpositions on the recirculation loop, the range of positions beingdefined between the downstream combustor and, proceeding in a downstreamdirection, the upstream combustor.
 29. The power plant according toclaim 28, wherein: the upstream combustor is configured to combine thecompressed oxidant from the oxidant compressor with the compressedexhaust gases from the recirculation compressor and, there within,combust the fuel from the upstream combustor fuel supply; and thedownstream combustor is configured to combine the compressed oxidantfrom the oxidant compressor with the exhaust gases from thehigh-pressure turbine and, there within, combust the fuel from thedownstream combustor fuel supply.
 30. The power plant according to claim28, further comprising means for testing the working fluid to determinewhether the downstream combustor is operating at the preferredstoichiometric ratio; wherein the means for testing the working fluid ispositioned on the recirculation loop relative to the predeterminedposition of the extraction point.
 31. The power plant according to claim30, the means for testing the working fluid comprises at least one of asensor for detecting excess oxidant and a sensor for detecting unspentfuel; and wherein the position of the means for testing the workingfluid on the recirculation loop comprises a range of positions, therange of positions being defined between the extraction point and,proceeding in an upstream direction, the downstream combustor.
 32. Thepower plant according to claim 15, wherein the at least one of theupstream combustor fuel supply and the downstream combustor fuel supplycomprises both of the upstream combustor fuel supply and the downstreamcombustor fuel supply; and wherein the means for controlling the powerplant such that one of the upstream combustor and the downstreamcombustor operates at the preferred stoichiometric ratio comprises meansfor controlling the power plant such that the upstream combustoroperates at the preferred stoichiometric ratio.
 33. The power plantaccording to claim 32, wherein the predetermined location of theextraction point comprises a range of positions on the recirculationloop, the range of positions being defined between the upstreamcombustor and, proceeding in a downstream direction, the downstreamcombustor.
 34. The power plant according to claim 33, wherein: theupstream combustor is configured to combine the compressed oxidant fromthe oxidant compressor with the compressed exhaust gases from therecirculation compressor and, there within, combust the fuel from theupstream combustor fuel supply; and the downstream combustor isconfigured to combine the compressed oxidant from the oxidant compressorwith the exhaust gases from the high-pressure turbine and, there within,combust the fuel from the downstream combustor fuel supply.
 35. Thepower plant according to claim 33, further comprising means for testingthe working fluid to determine whether the upstream combustor isoperating at the preferred stoichiometric ratio; wherein the means fortesting the working fluid is positioned on the recirculation looprelative to the predetermined position of the extraction point.
 36. Thepower plant according to claim 35, the means for testing the workingfluid comprises at least one of an oxygen sensor and a sensor fordetecting unspent fuel; and wherein the position of the means fortesting the working fluid on the recirculation loop comprises a range ofpositions, the range of positions being defined between the extractionpoint and, proceeding in an upstream direction, the upstream combustor.37. The power plant according to claim 15, further comprising an oxygensensor configured to test the working fluid of the recirculation loop,the oxygen sensor disposed between the extraction point and, proceedingin an upstream direction on the recirculation loop, the first of theupstream combustor and the downstream combustor encountered; furthercomprising means for determining whether the oxygen content exceeds apredetermined threshold.
 38. The power plant according to claim 2,further comprising: a load; and a common shaft that connects the load,the oxidant compressor, the recirculation compressor, the high-pressureturbine and the low-pressure turbine such that the high-pressure turbineand the low-pressure turbine drive the load, the oxidant compressor, andthe recirculation compressor.
 39. The power plant according to claim 38,wherein: the load comprises a generator; on the common shaft, therecirculation compressor resides between the high-pressure turbine andthe oxidant compressor; and on the common shaft, the high-pressureturbine resides between the low-pressure turbine and the recirculationcompressor.
 40. The power plant according to claim 2, furthercomprising: a generator; and concentric shafts including a first shaftand a second shaft; wherein the first shaft connects to thehigh-pressure turbine and drives at least one of the generator, theoxidant compressor, and the recirculation compressor; and wherein thesecond shaft connects to the low-pressure turbine and drives at leastone of the generator, the oxidant compressor, and the recirculationcompressor.
 41. The power plant according to claim 2, wherein the meansof extracting the portion of the working fluid comprises an extractionvalve that is configured to controllably vary a working fluid amountthat is extracted.
 42. The power plant according to claim 2, furthercomprising a recirculation conduit valve configured to vent acontrollable amount of working fluid to atmosphere; wherein therecirculation conduit valve comprises a position on the recirculationconduit.